Generate default shaders' OS specific versions to separate files to
avoid mtl_default_shaders_autogen.inc becoming too big.

Bug: angleproject:2634
Change-Id: Ie2a39971ec46c36ce13bf5cb72f998f2cbc0ab64
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2332145
Commit-Queue: Le Hoang Quyen <le.hoang.q@gmail.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>