[ARM] Use bitmap to control cmse_nonsecure_call register clearing

[ARM] Use bitmap to control cmse_nonsecure_call register clearing

As part of r253256, cmse_nonsecure_entry_clear_before_return has been
rewritten to use auto_sbitmap instead of an integer bitfield to control
which register needs to be cleared. This commit continues this work in
cmse_nonsecure_call_clear_caller_saved.

2017-11-22  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/arm.c (cmse_nonsecure_call_clear_caller_saved): Use
    auto_sbitap instead of integer bitfield to control register needing
    clearing.

From-SVN: r255055