libcc1: avoid extra string copies

PR c/94669 points out that a couple of spots in libcc1 take a
std::string where a reference would do.  This changes these spots to
take a const char *, to reduce the number of copies.

libcc1

	PR c/94669
	* compiler.hh (compiler_driver_filename): Take const char *.
	(compiler_triplet_regexp): Likewise.