ispc: 1.10.0 -> 1.13.0, cleanup, (co)maintain

This includes several enhancements in the underlying compiler, including
codegen improvements for AVX-512, Ice Lake CPU definitions,
cross-{arch,os} compilation (currently unsupported due to multilib
issues), and more.

This also bumps the LLVM backend to the 10.0 release. Note that ispc
itself requires a few extra stability patches on top of 10.0 for AVX-512
support, but these aren't applied for us. Therefore AVX-512 still has
some extra, rough edges.

Signed-off-by: Austin Seipp <aseipp@pobox.com>