Naming 'anonymous' mmap memory, which we use for Reactor's executable
memory, enables profilers to attribute JIT-compiled routine execution
to SwiftShader.

Also rename `swiftshader_reactor_private_config` to
`swiftshader_llvm_reactor_private_config` to clarify that it is only used
for building Reactor with the LLVM backend.

Bug: b/174801963
Change-Id: I4a6cb471b00f8403978870fd0be4608ee2798e54
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52208
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>