fenv.h: stop declaring feclearexcept() extern inline

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
Reviewed by: dim (x86)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D55975