Remove checks for __CC_SUPPORTS__INLINE assuming it is always true.

[jhb]

Summary

All supported compilers (modern versions of GCC and clang) support
this.

[imp]

IIRC, this would prevent tcc from compiling the kernel.
However, there are a lot of other things preventing that today, so the change is fine.
There may be some userland programs left over from the days of yore for i386 that might also be affected by alternate compilers, but it's more something to be aware of than something we can do anything about.
We assume __inline is a thing almost everywhere, so it isn't really buying us much.

[brooks]

We compile as for C99 so inline is supported and all modern compilers support __inline.