Update cmsis_gcc.h to undo removed __PKHBT and __PKHTB intrinsics

[etheory]

By commenting out PKHBT and PKHTB assembly versions, and instead using native C versions, you have absolutely crippled the performance of this header when using these instructions. In real world high performance code, I got a 6.58% speed improvement for the entire codebase JUST by making this change.

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