[ARM] Do softfloat when -mfpu set, -mfloat-abi=softfp

[ARM] Do softfloat when -mfpu set, -mfloat-abi=softfp

FP instructions are only enabled for TARGET_32BIT and TARGET_HARD_FLOAT
but GCC only gives an error when TARGET_HARD_FLOAT is true and -mfpu is
not set. Among other things, it makes some of the cmse tests (eg.
gcc.target/arm/cmse/baseline/softfp.c) fail when targeting
-march=armv8-m.base -mcmse -mfpu=<something> -mfloat-abi=softfp. This
commit adds an extra check for TARGET_32BIT to TARGET_HARD_FLOAT such
that it is false on TARGET_THUMB1 targets even when a FPU is specified.

2018-12-19  thomas Preud'homme  <thomas.preudhomme@linaro.org>

    gcc/
    * config/arm/arm.h (TARGET_HARD_FLOAT): Restrict to TARGET_32BIT
    targets.
    * config/arm/arm.c (output_return_instruction): Only check
    TARGET_HARD_FLOAT to decide whether FP instructions are available.

    gcc/testsuite/
    * gcc.target/arm/cmse/baseline/softfp.c: Force an FPU.

From-SVN: r267270