cppgc: Limit stack scan to clang-only

Quickfix to get non-clang bots green.

Bug: v8:10370
Change-Id: I83591cd2bda94cd5aaffd023fe2b7068bdc7442f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2131066Reviewed-by: Anton Bikineev <bikineev@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66928}

src/heap/cppgc/stack.h

@@ -8,9 +8,11 @@
 #include "src/base/macros.h"
 
 // TODO(chromium:1056170): Implement all platforms.
-#if defined(V8_TARGET_ARCH_X64)
+#ifdef __clang__
+#if defined(V8_HOST_ARCH_X64)
 #define CPPGC_SUPPORTS_CONSERVATIVE_STACK_SCAN 1
 #endif
+#endif
 
 namespace cppgc {
 namespace internal {