Support debug info in new ELF linker

[mlugg]

//! empty.zig
pub fn main() void {}

Build with old and new self-hosted ELF linkers, and compare the section headers:

$ zig build-exe empty.zig -fno-new-linker
$ readelf -WS empty
There are 28 section headers, starting at offset 0x2b0:

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .rodata           PROGBITS        0000000001001000 0c5000 003540 00   A  0   0 64
  [ 2] .rodata.cst       PROGBITS        0000000001004540 0c8540 0007c0 04  AM  0   0 32
  [ 3] .rodata.str       PROGBITS        0000000001004d00 0c8d00 000007 01 AMS  0   0  1
  [ 4] .eh_frame         X86_64_UNWIND   0000000001004d08 0c8d08 014978 00   A  0   0  8
  [ 5] .eh_frame_hdr     PROGBITS        0000000001019680 0dd680 000008 00   A  0   0  4
  [ 6] .text             PROGBITS        000000000101a000 5fc000 1a123a 00  AX  0   0 16
  [ 7] .tdata            PROGBITS        00000000011bc000 03a000 000008 00 WAT  0   0  4
  [ 8] .tbss             NOBITS          00000000011bc008 000000 000008 00 WAT  0   0  8
  [ 9] .data             PROGBITS        00000000011bd000 03b000 005298 00  WA  0   0 4096
  [10] .data.rel.ro      PROGBITS        00000000011c22a0 0402a0 0110f1 00  WA  0   0 32
  [11] .bss              NOBITS          00000000011d33c0 000000 001040 00  WA  0   0 64
  [12] .debug_abbrev     PROGBITS        0000000000000000 0017e0 0008df 00      0   0  1
  [13] .debug_aranges    PROGBITS        0000000000000000 0062c0 005d20 00      0   0 16
  [14] .debug_info       PROGBITS        0000000000000000 3b9385 0f323f 00      0   0  1
  [15] .debug_line       PROGBITS        0000000000000000 4bf455 084ebe 00      0   0  1
  [16] .debug_line_str   PROGBITS        0000000000000000 0032fe 001410 01  MS  0   0  1
  [17] .debug_loc        PROGBITS        0000000000000000 5707fd 0680bc 00      0   0  1
  [18] .debug_loclists   PROGBITS        0000000000000000 0015aa 000040 00      0   0  1
  [19] .debug_pubnames   PROGBITS        0000000000000000 02429c 003838 00      0   0  1
  [20] .debug_pubtypes   PROGBITS        0000000000000000 004dbe 000c25 00      0   0  1
  [21] .debug_ranges     PROGBITS        0000000000000000 028d91 00c520 00      0   0  1
  [22] .debug_rnglists   PROGBITS        0000000000000000 01bf3a 00628a 00      0   0  1
  [23] .debug_str        PROGBITS        0000000000000000 070a8c 03eea7 01  MS  0   0  1
  [24] .comment          PROGBITS        0000000000000000 039466 00001d 01  MS  0   0  1
  [25] .symtab           SYMTAB          0000000000000000 0e58b8 014c28 18     27 3543  8
  [26] .shstrtab         STRTAB          0000000000000000 03948c 00013b 01      0   0  1
  [27] .strtab           STRTAB          0000000000000000 101398 015e6a 01      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), l (large), p (processor specific)
$ ./stage4/bin/zig build-exe empty.zig -fnew-linker
$ readelf -WS empty
There are 8 section headers, starting at offset 0x208:

Section Headers:
  [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            0000000000000000 000000 000000 00      0   0  0
  [ 1] .symtab           SYMTAB          0000000000000000 022000 0174c0 18      3 3976  8
  [ 2] .shstrtab         STRTAB          0000000000000000 001000 001000 01      0   0 4096
  [ 3] .strtab           STRTAB          0000000000000000 003000 017000 01      0   0 4096
  [ 4] .rodata           PROGBITS        000000000101d000 01d000 005000 00   A  0   0 4096
  [ 5] .text             PROGBITS        0000000001077000 077000 2c5000 00  AX  0   0 4096
  [ 6] .data             PROGBITS        00000000019cf000 9cf000 027000 00  WA  0   0 4096
  [ 7] .tdata            PROGBITS        00000000019f6000 9f6000 001000 00 WAT  0   0 4096
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  D (mbind), l (large), p (processor specific)
$

The new linker has not emitted any DWARF sections (.debug_info etc).

It should emit the .debug_* sections when DWARF debug information is enabled (which it is by default in Debug mode).

CFI

DWARF's .debug_frame section, which holds Call Frame Information (CFI) required for (among other things) stack unwinding, is a little different. When using the self-hosted backend, CFI (CIEs and FDEs) should be emitted into:

• .eh_frame, if -fno-unwind-tables is not passed
• .debug_frame, if -fno-unwind-tables is passed and debug information is enabled
• Not emitted, if -fno-unwind-tables is passed and debug information is being stripped

A .eh_frame_hdr section should be emitted if there are any input .eh_frame sections, unless --no-eh-frame-hdr is passed.