Enable runtimes and ccache on Solaris builders (#965)

The Solaris builders currently default to building `compiler-rt` as a
project. This was necessary to avoid the `builtins` tests being skipped,
which has since been fixed. Besides, the projects build is deprecated.
Therefore this patch switches this. To avoid test failures, the
Solaris/amd64 `LLVM_HOST_TRIPLE` needs to use the canonical
`x86_64-pc-solaris2.11` name for the target.

At the same time, this patch anables the use of `ccache`.

Both changes have been tested manually on `sparcv9-sun-solaris2.11` and
`x86_64-pc-solaris2.11`.