[Driver] Use libatomic for 32-bit SPARC atomics support

Even after D86621 <https://reviews.llvm.org/D86621>, `clang -m32` on
Solaris/sparcv9 doesn't inline atomics with 8-byte operands, unlike `gcc`.
This leads to many link failures in the testsuite (undefined references to
`__atomic_load_8` and `__sync_val_compare_and_swap_8`.  Until a proper
codegen fix can be implemented, this patch works around the first of those
by linking with `-latomic`.

Tested on `sparcv9-sun-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D118021