manual: Fix Texinfo warnings about improper node names.

A number of cross-references to the GCC info manual cause Texinfo
warnings; e.g.:

  ./creature.texi:11: warning: @xref node name should not contain `.'

This is due to "gcc.info" being used in the INFO-FILE-NAME (fourth)
argument.  Changing it to "gcc" removes these warnings.  (Manually
confirmed equivalent behaviour for make info, html, and pdf.)

	* manual/creature.texi: Convert references to gcc.info to gcc.
	* manual/stdio.texi: Likewise.
	* manual/string.texi: Likewise.