Allow SwiftShader to compile on Fuchsia

Allow SwiftShader to compile on Fuchsia

SwiftShader should be able to build on Fuchsia, but it would
hit a collision error if added as a dependency in Fuchsia,
so the same logic currently used on MacOS could also be used
to avoid the collision on Fuchsia.

Change-Id: I9ac216dfc2180165a82bd5ae81c843ee4aea7ce6
Reviewed-on: https://swiftshader-review.googlesource.com/19328
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>