Allocate executable memory backed by named mmaps on Linux.

Allocate executable memory backed by named mmaps on Linux.

Executable heap memory can confuse profiling tools. Use memfd_create
on Linux, if possible, to create a named anonymous memory region.

Only enabled if LINUX_ENABLE_NAMED_MMAP is defined.

Bug b/73721724

Change-Id: I420711e4f64725ae834ab54264038683e4c445fe
Reviewed-on: https://swiftshader-review.googlesource.com/17208
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Ian Rogers <irogers@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>