[GOLD] Reduce size of class Symbol

On 64-bit targets there is a 32-bit hole in symbol->u_, and another
due to symbol flags exceeding 32 bits.  By splitting the union,
the total size of the class reduces by one 64-bit word.

	* symtab.h (Symbol): Split u_ into u1_ and u2_.  Adjust accessors
	to suit.  Move plt_offset_ before got_offsets_.
	* symtab.cc (Symbol::init_fields): Adjust for union change.
	(Symbol::init_base_output_data): Likewise.
	(Symbol::init_base_output_segment): Likewise.
	(Symbol::allocate_base_common): Likewise.
	(Symbol::output_section): Likewise.
	(Symbol::set_output_section): Likewise.
	(Symbol::set_output_segment): Likewise.
	* resolve.cc (Symbol::override_base): Likewise.
	(Symbol::override_base_with_special): Likewise.