This adds a general target for SPIR-V compilation. Previously there was not
any target machine defined for SPIR-V.
TODO is to reword the features for this target. We don't really need the full
list of capabilities in the features, we should only put a few features here
which we can actually use during code generation.
The idea is to depend on this language feature as little as possible
with the hopes that it can be adjusted to be less of an anti-pattern.
This also helps self-hosted, which does not yet implement
`usingnamespace`, get closer to being able to build compiler-rt.
