gas warning fixes

gas warning fixes

Some versions of gcc with -Werror=format-overflow complain about using
a perfectly good 7 char buffer for "r%dr%d" when the int is between 0
and 64, apparently not seeing the value range.

note: __builtin___sprintf_chk output between 5 and 24 bytes into a destination of size 7

	* config/tc-arc.c (declare_register_set): Avoid false positive
	format-overflow warning.
	* config/tc-epiphany.c (md_assemble): Likewise.
	* config/tc-mips.c (md_begin): Likewise.
	* config/tc-mmix.c (mmix_md_begin): Likewise.
	* config/tc-nds32.c (nds32_elf_append_relax_relocs): Avoid false
	positive "may be used uninitialized" warning.