[Driver] Fix linking with -lm on Solaris (#65632)

As noticed in D158846, the Solaris driver deviates from other targets in
that it links every executable with `-lm`, but doesn't for shared
objects. For C code, this is unnecessary, while for C++ `libm` is always
needed, even for shared objects.

This patch fixes this by following the `Gnu.cpp` precedent. It adjusts
the `solaris-ld.c` test accordingly, adding some more tests.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.