* ldmain.h (ld_sysroot): Change type to a constant string.

	* ldmain.h (ld_sysroot): Change type to a constant string.
	* ldmain.c (ld_sysroot): Likewise.
	(get_relative_sysroot, get_sysroot): New functions, adding command-line
	support for changing the sysroot.
	(main): Call the new functions.
	* lexsup.c (OPTION_SYSROOT): New.
	(ld_options): Add --sysroot.
	(parse_args): Add a dummy handler for it.
	* ld.texinfo (--sysroot): Document.
	* NEWS: Mention the new --sysroot option.