Adapt atomic compare and swap to ARMv8-M Baseline

Adapt atomic compare and swap to ARMv8-M Baseline

2016-10-26  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/sync.md (atomic_compare_and_swap<mode>_1): Add new ARMv8-M
    Baseline only alternatives to (i) hold store atomic success value in a
    return register rather than a scratch register, (ii) use a low register
    for it and to (iii) ensure the cbranchsi insn generated by the split
    respect the constraints of Thumb-1 cbranchsi4_insn and
    cbranchsi4_scratch.
    * config/arm/thumb1.md (cbranchsi4_insn): Add comment to indicate
    constraints must match those in atomic_compare_and_swap.
    (cbranchsi4_scratch): Likewise.

From-SVN: r241578