Remove per-architecture checks for enabling HAVE_FENV_H

The conditional came from NetBSD, where only select architectures have this
header/support

All architectures on FreeBSD have the necessary support though, so the
conditional's completely unnecessary

make tinderbox done on all architectures (including arm64, where the issue
occurred before) this time

X-MFC with: r282057, r282092
MFC after: 6 days