[SPIR-V] Add SPIRV to LLVM_ALL_TARGETS (reapply) (#123733)
This commit promotes the SPIR-V backend from experimental to official
status. As a result, SPIR-V will be built by default, simplifying
integration and increasing accessibility for downstream projects.
Discussion and RFC on Discourse:
https://discourse.llvm.org/t/rfc-promoting-spir-v-to-an-official-target/83614
The PR reapplies the original patch
https://github.com/llvm/llvm-project/pull/119653 and consecutive
https://github.com/llvm/llvm-project/pull/123654, reverted due to
buildbot failures.
