make a few variables static

They are only used in one file, so we might as well restrict there scope to
that file, and theoretically this might slightly improve compilers ability to
optimize usage of these variables.

gas/ChangeLog:

2016-04-14  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-nios2.c (nios2_as_options): Make file static.
	* config/tc-ppc.c (toc_reloc_ypes): Likewise.
	* config/tc-sparc.c (native_op_table): Likewise.