Support ld.mold linker.

gcc/ChangeLog:

	* collect2.c (main): Add ld.mold.
	* common.opt: Add -fuse-ld=mold.
	* doc/invoke.texi: Document it.
	* gcc.c (driver_handle_option): Handle -fuse-ld=mold.
	* opts.c (common_handle_option): Likewise.