riscv64: Add missing (slightly modified) fenv.h from FreeBSD

* A few tips for future folks follows.
* fenv.h gets wrapped in our buildtools
* If anything in the arch fenv.h "doesn't work" buildtools
will silently fail early on (autotools HAVE_FENV_H)

Change-Id: Icae064fde42af3bbed5ea2eadfaa8c18c677e6a6
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2164
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>