[libcxx] Adjust inline assembly constraints for the AMDGPU target (ll…

…vm#101747)

Summary:
These assembly constraints are illegal / invalid on the AMDGPU target.
The `r` constraint is only valid on inputs and the `m` constraint isn't
accepted at all. The NVPTX target can handle them because it uses a more
permissive virtual machine (PTX is an IR). Simply add exceptions on the
target to make these work.

libcxx/test/support/test_macros.h

@@ -291,17 +291,27 @@ struct is_same<T, T> { enum {value = 1}; };
 // when optimizations are enabled.
 template <class Tp>
 inline Tp const& DoNotOptimize(Tp const& value) {
-    asm volatile("" : : "r,m"(value) : "memory");
-    return value;
+  // The `m` constraint is invalid in the AMDGPU backend.
+#  if defined(__AMDGPU__) || defined(__NVPTX__)
+  asm volatile("" : : "r"(value) : "memory");
+#  else
+  asm volatile("" : : "r,m"(value) : "memory");
+#  endif
+  return value;
 }
 
 template <class Tp>
 inline Tp& DoNotOptimize(Tp& value) {
-#if defined(__clang__)
+  // The `m` and `r` output constraint is invalid in the AMDGPU backend as well
+  // as i8 / i1 arguments, so we just capture the pointer instead.
+#  if defined(__AMDGPU__)
+  Tp* tmp = &value;
+  asm volatile("" : "+v"(tmp) : : "memory");
+#  elif defined(__clang__)
   asm volatile("" : "+r,m"(value) : : "memory");
-#else
+#  else
   asm volatile("" : "+m,r"(value) : : "memory");
-#endif
+#  endif
   return value;
 }
 #else