Add new NOCROSSREFS_TO linker script command

NOCROSSREFS_TO is similar to the existing NOCROSSREFS command but only
checks one direction of cross referencing.

ld/ChangeLog

	* ld.texinfo: Document NOCROSSREFS_TO script command.
	* ldlang.h (struct lang_nocrossrefs): Add onlyfirst field.
	(lang_add_nocrossref_to): New prototype.
	* ldcref.c (check_local_sym_xref): Use onlyfirst to only look for
	symbols defined in the first section.
	(check_nocrossref): Likewise.
	* ldgram.y (NOCROSSREFS_TO): New script command.
	* ldlang.c (lang_add_nocrossref): Set onlyfirst to FALSE.
	(lang_add_nocrossref_to): New function.
	* ldlex.l (NOCROSSREFS_TO): New token.
	* NEWS: Mention NOCROSSREFS_TO.
	* testsuite/ld-scripts/cross4.t: New file.
	* testsuite/ld-scripts/cross5.t: Likewise.
	* testsuite/ld-scripts/cross6.t: Likewise.
	* testsuite/ld-scripts/cross7.t: Likewise.
	* testsuite/ld-scripts/crossref.exp: Run 4 new NOCROSSREFS_TO
	tests.