libclc: Add clspv64 target

Add a variant of the clspv target that is built using spir64.
This is a pre-requisite to supporting spir64 in clspv which is
required to take advantage of SPV_KHR_physical_storage_buffer which
in turn enables more OpenCL C programs to be compiled with clspv.

https://reviews.llvm.org/D116668