[Driver] Allow for sparcv8plus subdir with Solaris/SPARC GCC

Since GCC 11, the bundled Solaris/SPARC GCC uses the `sparcv8plus`
subdirectory for 32-bit objects, just like upstream GCC.  Before that, it
used `32` instead from a local patch.

Since `clang` doesn't know about that `sparcv8plus` subdirectory, it
wouldn't properly use GCC 11+ installations.

The new `solaris-sparc-gcc-search.test` testcase wasn't run initially (like
the existing `crash-report-null.test`) because the `.test` suffix wasn't
handled.

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

Differential Revision: https://reviews.llvm.org/D157013