Bug 1659057 - Allow hw.memsize system call. r=spohl

This fixes the following error when running on Apple Silicon DTK:

AGX: agxs_util.cpp:355:size_t getSystemMemorySize(): !!! Verification failed: status == 0

Differential Revision: https://phabricator.services.mozilla.com/D87048

security/sandbox/mac/SandboxPolicyContent.h

@@ -111,6 +111,7 @@ static const char SandboxPolicyContent[] = R"SANDBOX_LITERAL(
     ; removing it.
     (sysctl-name "kern.hostname")
     (sysctl-name "hw.machine")
+    (sysctl-name "hw.memsize")
     (sysctl-name "hw.model")
     (sysctl-name "hw.ncpu")
     (sysctl-name "hw.activecpu")