Update documentation of %X spec

Update documentation of %X spec

%X
Output the accumulated linker options specified by -Wl or a ‘%x’ spec string

The part about -Wl has been obsolete for 27 years, since this change:

Author: Torbjorn Granlund <tege@gnu.org>
Date:   Thu Oct 27 18:04:25 1994 +0000

    (process_command): Handle -Wl, and -Xlinker similar to -l,

    i.e., preserve their order with respect to linker input files.

Technically speaking, the arguments of -l, -Wl and -Xlinker are input files.

gcc/
	* doc/invoke.texi (%X): Remove obsolete reference to -Wl.