'linux' turns out to be a prebuilt macro for GCC (not Clang)
so using "namespace linux { ... }" actually does not compile.

This CL fixes the issue by removing the namespace, and using
Linux as a prefix for the corresponding class.

Bug: 140419396
Change-Id: I1e980bc66b2cf8f2e21d9f7076918e2b476ae4d4
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/37288
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: David Turner <digit@google.com>
Presubmit-Ready: David Turner <digit@google.com>