glsl: fix constant packing for 64-bit big endian.

In a piglit run on s390 a lot of double tests fail, explicitly
packing/shifting things rather than using memcpy seems to help

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5679>
2 files changed