llvmpipe: align scratch size to 64-bit size.

This fixes a crash with luxmark where it uses a 12-byte scratch space,
but when llvmpipe allocates it for 8 lanes, it isn't properly aligned
for 64-bit.

Karol found this debugging rusticl.

Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <!16288>
151 jobs for !16288 with llvmpipe-fix-scratch-align in 44 minutes and 26 seconds (queued for 19 seconds)
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