add more const qualifiers

add more const qualifiers

gas/ChangeLog:

2016-03-28  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/obj-elf.c (obj_elf_section_name): Return const char *.
	* config/obj-elf.h (obj_elf_section_name): Adjust.
	* config/tc-aarch64.c (aarch64_parse_features): Likewise.
	(aarch64_parse_cpu): Likewise.
	(aarch64_parse_arch): Likewise.
	* config/tc-arm.c (arm_parse_extension): Likewise.
	(arm_parse_cpu): Likewise.
	(arm_parse_arch): Likewise.
	* config/tc-nds32.c: Likewise.
	* config/xtensa-relax.c (parse_special_fn): Likewise.
	* stabs.c (generate_asm_file): Likewise.