spirv: Support big-endian strings

This should be all that's required for the SPIR-V parser to work in
big-endian systems.  SPIR-V requires that everything be in host
byte-order except for strings which are always little-endian.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6090>
2 files changed