fixup -Wshadow warnings on gcc-4.7

fixup -Wshadow warnings on gcc-4.7

gcc 4.7 complains about variables that shadow function names, which now happens
in tc-arm.c because there is a global function do_align (), and local variables
do_align.  The simplest fix for this seems to be to rename those variables to
do_alignment.

gas/ChangeLog:

2016-03-09  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-arm.c (neon_alignment_bit): Rename do_align to
	do_alignment.
	(do_neon_ld_st_lane): Likewise.
	(do_neon_ld_dup): Likewise.