[mlir][spirv] Allow custom mangling of SPIRV built-in global variables

The SPIR-V spec does not specify the mangling for these variables, so
the conversion to SPIR-V should be flexible enough to allow adding a
custom prefix and suffix to the core name.

Differential Revision: https://reviews.llvm.org/D153951



Signed-off-by: Victor Perez <victor.perez@codeplay.com>