Add missing fenv.h for sparc.

Borrowed from FreeBSD with some changes to get it building.
Now we need to rebuild the gcc package...

Change-Id: I6b8dfd7fb6ca912c76e2ff10fbe01ad583a09aec
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2131
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>