gate pragma behind defined(__clang__)

Change-Id: Ice32fabe79286ec6cf93987608cd989cf7b28e74

Author: mauri870

src/runtime/cgo/gcc_libinit.c

@@ -4,10 +4,12 @@
 
 //go:build unix
 
-// When cross-compiling to linux/armv5, atomics are emulated and cause a
-// compiler warning. This results in a build failure since cgo uses
-// -Werror. See #65290.
+// When cross-compiling with clang to linux/armv5, atomics are emulated
+// and cause a compiler warning. This results in a build failure since
+// cgo uses -Werror. See #65290.
+#if defined(__clang__)
 #pragma GCC diagnostic ignored "-Watomic-alignment"
+#endif
 
 #include <pthread.h>
 #include <errno.h>