These were left over from $FreeBSD$ removal.
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42612
(cherry picked from commit 1ca63a8219b88b752b064d19bd3428c61dbcf1f9)
The arm, mips, and riscv MD Symbol.map files listed some (but not all)
of the softfloat symbols that were actually defined in softfloat.c.
While here, also remove entries for __fixuns[sd]fsi which are provided
by libcompiler_rt and not by libc.
Sponsored by: DARPA / AFRL