ld --defsym

This makes --defsym support the same expressions as assignment in a
script.  For example, --defsym 'HIDDEN(foo=0)', will define a hidden
visibility foo.

	* ldgram.y (defsym_expr): Use assignment rule.
	* ldlex.h (ldlex_defsym): Delete.
	* ldlex.l (DEFSYMEXP, ldlex_defsym): Delete.