Update fuchsia sanitizer sched_yield

[gevalentino]

Summary

Fuchsia has split overloaded nanosleep(0) for yielding to its own dedicated syscall, so valid zero deadlines would just return.

[mcgrathr]

lgtm since the compatibility window for platform versions that didn't have the new entry point has already passed by now.

compiler-rt/lib/sanitizer_common/sanitizer_fuchsia.cpp

 #  include "sanitizer_libc.h"
 #  include "sanitizer_mutex.h"
 
 namespace __sanitizer {
 
 void NORETURN internal__exit(int exitcode) { _zx_process_exit(exitcode); }
 
 uptr internal_sched_yield() {
-  zx_status_t status = _zx_nanosleep(0);
+  zx_status_t status = _zx_thread_legacy_yield(0u);
   CHECK_EQ(status, ZX_OK);
   return 0;  // Why doesn't this return void?
 }
 
 void internal_usleep(u64 useconds) {
   zx_status_t status = _zx_nanosleep(_zx_deadline_after(ZX_USEC(useconds)));
   CHECK_EQ(status, ZX_OK);
 }