cmd/internal/obj/arm64: add support for PAC instructions

Add support for the Pointer Authentication Code instructions
required for the ELF ABI when enabling PAC aware binaries.

This allows for assembly writers to add PAC instructions where needed to
support this ABI. Follow up work is to enable the compiler to emit these
instructions in the appropriate places.

The TL;DR for the Linux ABI is that the prologue of a function that
pushes the link register (LR) to the stack, signs the LR with a key
managed by the operating system and hardware using a PAC instruction,
like "paciasp". The function epilog, when restoring the LR from the
stack will verify the signature, using an instruction like "autiasp".

This helps prevents attackers from modifying the return address on the
stack, a common technique for ROP attacks.

Details on PAC can be found here:
  - https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/enabling-pac-and-bti-on-aarch64
  - https://developer.arm.com/documentation/109576/0100/Pointer-Authentication-Code

The ABI details can be found here:
  - https://github.com/ARM-software/abi-aa/blob/main/aaelf64/aaelf64.rst

Change-Id: I4516ed1294d19f9ff9d278833d542821b6642aa9
Reviewed-on: https://go-review.googlesource.com/c/go/+/676675
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: billatarm

src/cmd/asm/internal/asm/testdata/arm64.s

@@ -1894,4 +1894,12 @@ next:
 	BTI	J                                  // 9f2403d5
 	BTI	JC                                 // df2403d5
 
+// Pointer Authentication Codes (PAC)
+	PACIASP                                    // 3f2303d5
+	AUTIASP                                    // bf2303d5
+	PACIBSP                                    // 7f2303d5
+	AUTIBSP                                    // ff2303d5
+	AUTIA1716                                  // 9f2103d5
+	AUTIB1716                                  // df2103d5
+
 	END

src/cmd/asm/internal/asm/testdata/arm64error.s

@@ -422,4 +422,10 @@ TEXT errors(SB),$0
 	SHA1H	V1.B16, V2.B16                                   // ERROR "invalid operands"
 	BTI                                       		 // ERROR "missing operand"
 	BTI	PLDL1KEEP					 // ERROR "illegal argument"
+	PACIASP	C                                                // ERROR "illegal combination"
+	AUTIASP	R2                                               // ERROR "illegal combination"
+	PACIBSP	R0                                               // ERROR "illegal combination"
+	AUTIBSP	C                                                // ERROR "illegal combination"
+	AUTIA1716	$45                                      // ERROR "illegal combination"
+	AUTIB1716	R0                                       // ERROR "illegal combination"
 	RET

src/cmd/internal/obj/arm64/a.out.go

@@ -1020,6 +1020,12 @@ const (
 	AWORD
 	AYIELD
 	ABTI
+	APACIASP
+	AAUTIASP
+	APACIBSP
+	AAUTIBSP
+	AAUTIA1716
+	AAUTIB1716
 	ALAST
 	AB  = obj.AJMP
 	ABL = obj.ACALL

src/cmd/internal/obj/arm64/anames.go

@@ -3017,6 +3017,13 @@ func buildop(ctxt *obj.Link) {
 			oprangeset(ANOOP, t)
 			oprangeset(ADRPS, t)
 
+			oprangeset(APACIASP, t)
+			oprangeset(AAUTIASP, t)
+			oprangeset(APACIBSP, t)
+			oprangeset(AAUTIBSP, t)
+			oprangeset(AAUTIA1716, t)
+			oprangeset(AAUTIB1716, t)
+
 		case ACBZ:
 			oprangeset(ACBZW, t)
 			oprangeset(ACBNZ, t)
@@ -7016,6 +7023,24 @@ func (c *ctxt7) op0(p *obj.Prog, a obj.As) uint32 {
 
 	case ASEVL:
 		return SYSHINT(5)
+
+	case APACIASP:
+		return SYSHINT(25)
+
+	case AAUTIASP:
+		return SYSHINT(29)
+
+	case APACIBSP:
+		return SYSHINT(27)
+
+	case AAUTIBSP:
+		return SYSHINT(31)
+
+	case AAUTIA1716:
+		return SYSHINT(12)
+
+	case AAUTIB1716:
+		return SYSHINT(14)
 	}
 
 	c.ctxt.Diag("%v: bad op0 %v", p, a)