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?