Avoid warnings from makeinfo

Avoid warnings from makeinfo

"make info" gives a number of warnings about the use of a "." in
@ref-like commands.  These come from the ".info" suffix.  I think this
suffix is redundant, and removing the suffix also removes the warning.

gdb/doc/ChangeLog
2018-09-10  Tom Tromey  <tom@tromey.com>

	* gdb.texinfo (Compilation): Use "gcc", not "gcc.info", in @xref.
	(Machine Code): Use "binutils", not "binutils.info", in @pxref.
	(Separate Debug Files): Use "ld", not "ld.info", in @ref.
	* python.texi (Objfiles In Python): Use "ld", not "ld.info", in @ref.