Fix duplicate output section statement lookup

Tie output section statements to their associated output section via
output section userdata.  This allows us to avoid hash lookups which
are slower and fail when multiple output sections have the same name.

	* ldlang.h (lang_output_section_get): Define.
	* ldlang.c (lang_output_section_get): Likewise.
	(init_os): Set the output_section userdata to the output
	section statement.
	* emultempl/hppaelf.em: Use lang_output_section_get instead of
	lang_output_section_find where applicable.
	* emultempl/aarch64elf.em: Likewise.
	* emultempl/aix.em: Likewise.
	* emultempl/armelf.em: Likewise.
	* emultempl/m68hc1xelf.em: Likewise.
	* emultempl/metagelf.em: Likewise.
	* emultempl/mipself.em: Likewise.
	* emultempl/ppc64elf.em: Likewise.
	* emultempl/spuelf.em: Likewise.