Refactor get_storage_spec

Return pointer rather than struct, which is very difficult to
JIT correctly due to ABI differences between windows an posix.

Doesn't work entirely because somehow we end up with a zeroed-
out storage spec for p6num. Will continue investigation tomorrow.
Create storagespecs as needed for P6num and P6int
Re-enable objprimspec
Fix unitialized use in P6int / P6num
Merge branch 'moar-jit'