Fix ICE on empty FIQ interrupt handler on ARM

2016-11-16  Thomas Preud'homme  <thomas.preudhomme@arm.com>

    gcc/
    * config/arm/arm.md (arm_addsi3): Add alternative for addition of
    general register with general register or ARM constant into SP
    register.

    gcc/testsuite/
    * gcc.target/arm/empty_fiq_handler.c: New test.

From-SVN: r242508