macOS Sandbox: Add psynch_rw_rdlock sycalls

This syscall is required by CRYPTO_BUFFER of BoringSSL.

Bug: 1050158
Change-Id: I3b14a263c81ab614005a8f879e64bf4b3634ad76
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143884Reviewed-by: Greg Kerr <kerrnel@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758457}

services/service_manager/sandbox/mac/common.sb

@@ -271,6 +271,7 @@
       (syscall-number SYS_proc_info)
       (syscall-number SYS_psynch_mutexdrop)
       (syscall-number SYS_psynch_mutexwait)
+      (syscall-number SYS_psynch_rw_rdlock)
       (syscall-number SYS_read)
       (syscall-number SYS_read_nocancel)
       (syscall-number SYS_readlink)