[TF-M] [Compiler related] Unify necessary and minimal set compiler definitions for SPM

Hi,

We created a proposal to define a minimal set of compiler specific-definitions for SPM. The reason is to avoid many #ifdef inside SPM code.

Only limited definitions are defined. Platform sources need to use platform defined headers for these definitions, such as CMSIS headers.

Special usage such as 'weak' or 'noreturn' are forbidden inside SPM.

Please put comments for this change: https://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/4211

Or reply here.

This is just an example patch, the follow up would apply this defined headers to all SPM sources.

Thanks

/Ken