m68k: really add fenv from musl-1.1.24

Somehow the first review merged only the commit log.

FreeBSD doesn't have m68k anyway, so use fenv from musl with as less
changes as possible.

Change-Id: I6372af6679e6773fbb6bf4c8b5b30512971a97a6
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3161
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>