unix: implement cpu_relax() for arm

Emits yield instruction in ARM mode (mainly needs to pass -march=...).

PR-URL: https://github.com/libuv/libuv/pull/2751
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>

src/unix/atomic-ops.h

@@ -53,6 +53,8 @@ UV_UNUSED(static int cmpxchgi(int* ptr, int oldval, int newval)) {
 UV_UNUSED(static void cpu_relax(void)) {
 #if defined(__i386__) || defined(__x86_64__)
   __asm__ __volatile__ ("rep; nop");  /* a.k.a. PAUSE */
+#elif (defined(__arm__) && __ARM_ARCH >= 7) || defined(__aarch64__)
+  __asm__ volatile("yield");
 #endif
 }