Move support code for linker script DEFINED to ldexp.c

Move support code for linker script DEFINED to ldexp.c

This moves support code for DEFINED to ldexp.c where it is used,
losing the lang_ prefix on identifiers.  Two new functions are needed
to initialize and clean up to hash table, but other than that there
are no functional changes here.

	* ldexp.c (struct definedness_hash_entry, definedness_table)
	(definedness_newfunc, symbol_defined, update_definedness): Move
	and rename from..
	* ldlang.h (struct lang_definedness_hash_entry): ..here,..
	* ldlang.c (lang_definedness_table, lang_definedness_newfunc)
	(lang_symbol_defined, lang_update_definedness): ..and here.
	* ldexp.c (ldexp_init, ldexp_finish): New functions, extracted from..
	* ldlang.c (lang_init, lang_finish): ..here.
	* ldexp.h (ldexp_init, ldexp_finish): Declare.
	* ldlang.h (lang_symbol_defined, lang_update_definedness): Delete.
	* ldmain.c (main): Call ldexp_init and ldexp_finish.