[Driver][SYCL] Use only PATH when searching for user host compiler

[mdtoguchi]

When using -fsycl-host-compiler, be sure that we only use the available PATH settings to find the compiler given (if given without a full PATH qualifier). This allows for only known locations to be searched for the given compiler instead of possible internal settings unknown to the user.

[mdtoguchi]

@intel/llvm-gatekeepers, this is ready for merge, thanks!