C library: __asm("<X>fence") is x86-only

Builds on Mac M1 were failing when (rightly) reporting that these were
unrecognised mnemonics.

Author: tautschnig

src/ansi-c/library/gcc.c

@@ -1,24 +1,36 @@
 /* FUNCTION: __builtin_ia32_sfence */
 
+#if defined(__i386__) || defined(__x86_64__)
+
 void __builtin_ia32_sfence(void)
 {
   __asm("sfence");
 }
 
+#endif
+
 /* FUNCTION: __builtin_ia32_lfence */
 
+#if defined(__i386__) || defined(__x86_64__)
+
 void __builtin_ia32_lfence(void)
 {
   __asm("lfence");
 }
 
+#endif
+
 /* FUNCTION: __builtin_ia32_mfence */
 
+#if defined(__i386__) || defined(__x86_64__)
+
 void __builtin_ia32_mfence(void)
 {
   __asm("mfence");
 }
 
+#endif
+
 /* FUNCTION: __sync_synchronize */
 
 void __sync_synchronize(void)