[gold] Implement HIDDEN syntax in linker scripts

gold/
	* yyscript.y (HIDDEN): New %token.
	(assignment): Handle HIDDEN(string = expr) syntax.
	* script.cc (script_keyword_parsecodes): Add HIDDEN.

(cherry picked from commit f300578f78f5ecddc9cfb28c02d032a0acffdf70)
3 files changed