[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

compiler-rt/lib/sanitizer_common/sanitizer_atomic_clang.h

@@ -74,13 +74,12 @@ template <typename T>
 inline bool atomic_compare_exchange_strong(volatile T *a, typename T::Type *cmp,
                                            typename T::Type xchg,
                                            memory_order mo) {
-  typedef typename T::Type Type;
-  Type cmpv = *cmp;
-  Type prev;
-  prev = __sync_val_compare_and_swap(&a->val_dont_use, cmpv, xchg);
-  if (prev == cmpv) return true;
-  *cmp = prev;
-  return false;
+  // Transitioned from __sync_val_compare_and_swap to support targets like
+  // SPARC V8 that cannot inline atomic cmpxchg.  __atomic_compare_exchange
+  // can then be resolved from libatomic.  __ATOMIC_SEQ_CST is used to best
+  // match the __sync builtin memory order.
+  return __atomic_compare_exchange(&a->val_dont_use, cmp, &xchg, false,
+                                   __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST);
 }
 
 template<typename T>