[sanitizer_common] Use atomic builtin in sanitizer_atomic_clang.h

As discussed in D118021 <https://reviews.llvm.org/D118021>, `clang -m32` on
Solaris/sparcv9 currently incorrectly doesn't inline atomics on 8-byte
operands, unlike `gcc`.  With the workaround in that patch in place, we're
left with may undefined references to `__sync_val_compare_and_swap_8`,
which isn't provided by `libatomic`.  This reference is due to the use of
`__sync_val_compare_and_swap` in `sanitizer_atomic_clang.h`'s
`atomic_compare_exchange_strong`.  As is already done in
`scudo/standalone/atomic_helpers.h`, using `__atomic_compare_exchange`
instead avoids this problem.

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

Differential Revision: https://reviews.llvm.org/D118024