This change restricts the effect of REACTOR_ANONYMOUS_MMAP_NAME to
platforms which define the __linux__ macro, since this feature makes
use of mmap(), syscall(), and posix_memalign().

Note Android is considered Linux.

Bug: b/174801963
Change-Id: Id194aca837b2c9c23f5c7419c926bc2872b77d1b
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/52209
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>