[libc] Add PAUSE instruction for the RPC spin loop

[jhuber6]

Summary

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.

[jhuber6]

Switch to builtin version

libc/src/__support/RPC/rpc_util.h

 constexpr uint64_t MAX_LANE_SIZE = 64;
 
 /// Suspend the thread briefly to assist the thread scheduler during busy loops.
 LIBC_INLINE void sleep_briefly() {
 #if defined(LIBC_TARGET_ARCH_IS_NVPTX) && __CUDA_ARCH__ >= 700
   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
 }
 
 /// Get the first active thread inside the lane.
 LIBC_INLINE uint64_t get_first_lane_id(uint64_t lane_mask) {
   return __builtin_ffsl(lane_mask) - 1;