Support arm compiler 5

[mathieudinel]

in compiler files there is a direct use of built-in functions when GNUC is defined. I use Arm Compiler 5 with --gnu flag along with C99 and the compiler defines GNUC and falls into armgcc territory. Would it be possible to support ARMCC by leveraging cmsis instead of directly invoking gcc builtins? I understand that not all gcc ports are ARM based, that's probably why it's currently written the way it is... I see at least one built in function is patched with freertos, but not when no-os is selected? maybe the patch would be better off in the compilers section?

[mathieudinel]

I feel left out here....

I was able to externally patch (through tusb_config.h) most of the incompatibilities, but I have stumbled across a major one and it requires a change in the source files.

Is there any plans to support Arm Compiler 5 even if the compiler itself is no longer maintained?

[hathach]

how to obtain the compiler and does it requires a license to use ?

[mathieudinel]

it does require a license. the compiler can be obtained from Keil.com, and I know at least one chip vendor has personal-use limited licenses for some of their chips. (ST has something for cortex M0+).
Unlicensed, the linker will limit to 32kb firmwares, not sure if this is sufficient for the many code examples?

As mentioned in #2537 It appears as though the project is slowly migrating to another syntax for weak function definitions. this would be the way to go.

[mathieudinel]

The changes are pretty straightforward, I will consider opening a PR, given my employer grants me permission to do so.