Remove most of c99_compat

Corentin Noël requested to merge tintou/c11-cleanups into master

As Mesa itself, we now rely on c11, let's remove most of the c99_compat.

Remove c99_math.h and needless c99_compat includes, it is still around for the sole reason of supporting the restrict-keyword from C++ code.

Based on mesa/mesa!16812 (merged)

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