libclc: Add Mesa/SPIR-V target

Add targets to emit SPIR-V targeted to Mesa's OpenCL support, using
SPIR-V 1.1.

Substantially based on Dave Airlie's earlier work.

libclc: spirv: remove step/smoothstep apis not defined for SPIR-V

libclc: disable inlines for SPIR-V builds

Reviewed By: jvesely, tstellar, jenatali

Differential Revision: https://reviews.llvm.org/D77589