Bug 257985

In Bug 256728, I attempted to enable `SYS_faccessat` in the iOS sandbox, as we had telemetry indicating it was getting hit. Unfortunately, this caused a memory benchmark to regress by a small amount. Without a strong reason to keep the feature accessible, we decided to silence it to avoid the cost of symbolicating telemetry and reports. Since then, I have discovered that some aspects of Metal's shader caching needs this value. This patch restores the feature. I will work to determine whether this is a true memory regression, or simply reverting a mistaken memory progression created when we blocked it in our April code release.