[Driver] Add crtfastmath.o on Solaris if appropriate

`Flang :: Driver/fast_math.f90` `FAIL`s on Solaris because `crtfastmath.o`
is missing from the link line.

This patch adds it as appropriate.

Tested on `amd64-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D141596