boot/util: use x86 ASM only on x86 platforms.

src/boot/efi/util.c

@@ -33,7 +33,7 @@ UINT64 ticks_read(VOID) {
         __asm__ volatile ("rdtsc" : "=a" (a), "=d" (d));
         return (d << 32) | a;
 }
-#else
+#elif defined(__i386__)
 UINT64 ticks_read(VOID) {
         UINT64 val;
         __asm__ volatile ("rdtsc" : "=A" (val));