MIPS/GAS/test: Fix an n32 `.reginfo' size test failure

MIPS/GAS/test: Fix an n32 `.reginfo' size test failure

Correct a commit 2d6dda71611b ("MIPS/BFD: Correctly report unsupported
`.reginfo' section size") issue and avoid a GAS test failure:

regexp_diff match failure
regexp "^.*: Incorrect `\.reginfo' section size; expected 24, got 28$"
line   "../as-new: dump.o: Incorrect `.reginfo' section size; expected 24, got 32"
FAIL: MIPS assembled .reginfo section size (n32)

on MIPS targets other than bare-metal ones.  The reason for this failure
is section padding to alignment, done in `size_seg'.  For n32 `.reginfo'
the section alignment is set to 3, and therefore the section is padded
to a multiple of 8, except for bare-metal targets, for which padding is
unconditionally disabled in `md_section_align'.

Use `--no-pad-sections' then to disable padding for all targets, so that
the size of `.reginfo' is always the same, matching the message pattern.

	gas/
	* testsuite/gas/mips/reginfo-2-n32.d: Add `--no-pad-sections' to
	`as' flags.