constraints.md (Q constraint): Document its use for Thumb-1.

constraints.md (Q constraint): Document its use for Thumb-1.

2016-10-25  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/constraints.md (Q constraint): Document its use for
    Thumb-1.
    (Pf constraint): New constraint for relaxed, consume or relaxed memory
    models.
    * config/arm/sync.md (atomic_load<mode>): Add new ARMv8-M Baseline only
    alternatives to allow any register when memory model matches Pf and
    thus lda is used, but only low registers otherwise.  Use unpredicated
    output template for Thumb-1 targets.
    (atomic_store<mode>): Likewise for stl.
    (arm_load_exclusive<mode>): Add new ARMv8-M Baseline only alternative
    whose output template does not have predication.
    (arm_load_acquire_exclusive<mode>): Likewise.
    (arm_load_exclusivesi): Likewise.
    (arm_load_acquire_exclusivesi): Likewise.
    (arm_store_release_exclusive<mode>): Likewise.
    (arm_store_exclusive<mode>): Use unpredicated output template for
    Thumb-1 targets.

From-SVN: r241507