[SPIR-V Backend][SPIR Triples] Align SPIR target triples between SPIR-V Backend and SYCL/Clang frontend

Upstream have started to add SPIRV target that register spirv64.

```
extern "C" LLVM_EXTERNAL_VISIBILITY void LLVMInitializeSPIRVTargetInfo() {
RegisterTarget X(getTheSPIRV32Target(), "spirv32",
"SPIR-V 32-bit", "SPIRV");
RegisterTarget Y(getTheSPIRV64Target(), "spirv64",
"SPIR-V 64-bit", "SPIRV");
RegisterTarget Z(getTheSPIRVLogicalTarget(), "spirv",
"SPIR-V Logical", "SPIRV");
}
```

There are some warnings emitted when the user/driver specifies 'spir/spir64/spir32' as the triples.
It will be useful to align on how to interpret these triples.

Thanks

Start at LLVMInitializeSPIRVTargetInfo and compare how the SPIR-V backend and SYCL/Clang frontend interpret the spir, spir32, and spir64 triples. The work is done when their interpretations are aligned and the warnings for these user- or driver-specified triples are resolved.

Written by the indexing model from the issue text.