[asm][ie86-x86-64] Fix build error (llvm#164532)

Fixes llvm#164453.

Author: googlewalt

compiler-rt/lib/builtins/assembly.h

@@ -337,7 +337,7 @@
 #endif
 #endif
 
-#if defined(__i386__) || defined(__amd64__)
+#if defined(__ASSEMBLER__) && (defined(__i386__) || defined(__amd64__))
 .att_syntax
 #endif