This makes it possible to build swiftshader for the host.

The target build also works but it has been disabled.

Change-Id: I8aecf931f4ebf72c06d4e3423e2eafea6bfa7f6c
Reviewed-on: https://swiftshader-review.googlesource.com/17988Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Alistair Strachan <astrachan@google.com>