Move softfloat symbol map entries to softfloat/Symbol.map.

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