fenv.h: stop declaring feclearexcept() extern inline

[kib]

Summary

The function is already exported from libm.  We only need to stop
declare it extern inline, and instead provide a macro which uses the
internal inline function __feclearexcept_int() instead.

PR:     277958

[kib]

I only handled x86 for start. After everybody is fine with the patch, I will add other arches.

[dim]

Yeah, this looks fine to me. (Though I got confused initially by the _int suffix, until I understood that it's shorthand for internal... I was mislead by the int argument)

[kib]

Handle all arches

[kib]

Do we need to handle the rest of the fenv functions same, they all are extern inline. Then we can remove the requirement to have gcc semantic for inline as a bonus.