[libc] Add PAUSE instruction for the RPC spin loop

The other architectures use a brief sleep to defer work during this spin
loop that checks the RPC mailboxes. This patch adds one for x64 to
improve usage when running the server.

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D156566

libc/src/__support/RPC/rpc_util.h

@@ -26,6 +26,8 @@ LIBC_INLINE void sleep_briefly() {
   LIBC_INLINE_ASM("nanosleep.u32 64;" ::: "memory");
 #elif defined(LIBC_TARGET_ARCH_IS_AMDGPU)
   __builtin_amdgcn_s_sleep(2);
+#elif defined(LIBC_TARGET_ARCH_IS_X86)
+  __builtin_ia32_pause();
 #else
   // Simply do nothing if sleeping isn't supported on this platform.
 #endif