msun/arm: convert fenv functions to proper linkage

namely
fegetexceptflag(3)
fesetexceptflag(3)
feraiseexcept(3)
fetestexcept(3)
fegetround(3)
fesetround(3)
fegetenv(3)
feholdexcept(3)
fesetenv(3)
feupdateenv(3)
feenableexcept(3)
fedisableexcept(3)
fegetexcept(3)

PR: 277958
Reviewed by: dim
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D56139