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

Also suppress a GCC warning about not using the result of ftruncate().

Bug: b/174801963
Bug: b/178614275
Change-Id: Iac4938f332f0e75a9b1d995c83f0cbaaf3532651
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52228Tested-by: Nicolas Capens <nicolascapens@google.com>
Commit-Queue: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>