On macOS 10.14 and higher, executables that are code signed with the
"runtime" option cannot execute writable memory by default. They can opt
into this capability by specifying the "com.apple.security.cs.allow-jit"
code signing entitlement and allocating the region with the MAP_JIT flag.

Note this change only affects use of the Subzero backend of Reactor.

Bug: chromium:985816
Change-Id: I7ea213cc2b62b64c85c4c46a0d4f28e0bace5d21
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34509Tested-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>