[ARM] Fix ICE in Armv8-M Security Extensions code

[ARM] Fix ICE in Armv8-M Security Extensions code

Commit r253825 which introduced some sanity checks for sbitmap revealed
a bug in the conversion of cmse_nonsecure_entry_clear_before_return ()
to using bitmap structure. bitmap_and expects that the two bitmaps have
the same length, yet the code in
cmse_nonsecure_entry_clear_before_return () have different size for
to_clear_bitmap and to_clear_arg_regs_bitmap, with the assumption that
bitmap_and would behave has if the bits not allocated were in fact zero.
This commit makes sure both bitmap are equally sized.

2017-11-17  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/arm.c (cmse_nonsecure_entry_clear_before_return): Allocate
    to_clear_arg_regs_bitmap to the same size as to_clear_bitmap.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@254859 138bc75d-0d04-0410-961f-82ee72b054a4