swr: [rasterizer common] os.h portability header changes
swr: [rasterizer common] os.h portability header changes

- Fix conflict between windows MemoryFence and llvm::sys::MemoryFence
- Declare gettid()

Signed-off-by: Tim Rowley <54947742fe7299a578dbb84f6c782c6c4c668d8f@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler