[SYCL] Enable compiler rt builtin for SPIRV device code

[jinge90]

This PR enables compiler-rt builtin library for SYCL device code. Currently, we only need __mulsc3/dc3 and __divsc3/dc3 for complex multiplication/division.

[jinge90]

Hi, @bader
We begin to enable compiler-rt builtin library for SYCL device code, this PR is the first one.
For compiler-rt builtin library, we only need 4 functions (__mulsc3, __muldc3, __divsc3, __divdc3) on Linux platform only.
We ported corresponding compiler-rt tests to SYCL e2e in #21622. This PR is still in progress, it only builds the libclang_rt.builtins.bc for device code, more work is needed to support linking it in compiler driver. I will ping you to review when all parts are finished.
Thanks very much.

[bader]

Hi, @bader We begin to enable compiler-rt builtin library for SYCL device code, this PR is the first one. For compiler-rt builtin library, we only need 4 functions (__mulsc3, __muldc3, __divsc3, __divdc3) on Linux platform only. We ported corresponding compiler-rt tests to SYCL e2e in #21622. This PR is still in progress, it only builds the libclang_rt.builtins.bc for device code, more work is needed to support linking it in compiler driver. I will ping you to review when all parts are finished. Thanks very much.

Hi @jinge90. Thanks for tagging me. I suggest we move this PR to https://github.com/llvm/llvm-project/ to review these changes with compiler-rt code owners.

A couple of high-level notes/questions:

1. I think we shouldn't switch from C to SYCL language compilation mode when we build for SPIR-V target. Have you tried using the C language mode?
2. Do you have any estimation how hard it would be to enable the generic sources list?