[SPIR-V] Prefer llvm-spirv-<LLVM_VERSION_MAJOR> tool (#77897)

Prefer using `llvm-spirv-<LLVM_VERSION_MAJOR>` tool (i.e.
`llvm-spirv-18`) over plain `llvm-spirv`. If the versioned tool is not
found in PATH, fall back to use the plain `llvm-spirv`.

An issue with the using `llvm-spirv` is that the one found in PATH might
be compiled against older LLVM version which could lead to crashes or
obscure bugs. For example, `llvm-spirv` distributed by Ubuntu links
against different LLVM version depending on the Ubuntu release (LLVM-10
in 20.04LTS, LLVM-13 in 22.04LTS).