[arm] Make -mfloat-abi=softfp work when there are no

Before this patch series it wasn't really possible to not have an FPU;
it was always there, even if the hardware didn't really support it.
Now that we have -mfpu=auto, the concept of not having an FPU becomes
real.  Consequently, when the -mfloat-abi switch is set to softfp
doing the Right Thing is much more important.  In this case we have a
soft-float ABI, but can use FP instructions if they are available.
To support this we have to separate out TARGET_HARD_FLOAT into two
use cases: one where the instructions exist and one when they don't.
We preserve the original meaning of TARGET_HARD_FLOAT (but add an extra
check) of meaning that we are generating HW FP instructions, and add a
new macro for the special case when use of FP instructions is permitted,
but might not be available at this time (the distinction is important
because they might be enabled by an attribute during the compilation).
TARGET_SOFT_FLOAT continues to be the exact inverse of TARGET_HARD_FLOAT,
but we now define it as such.

	* config/arm/arm.h (TARGET_HARD_FLOAT): Also check that we
	have some floating-point instructions.
	(TARGET_SOFT_FLOAT): Define as inverse of TARGET_HARD_FLOAT.
	(TARGET_MAYBE_HARD_FLOAT): New macro.
	* config/arm/arm-builtins.c (arm_init_builtins): Use
	TARGET_MAYBE_HARD_FLOAT.
	* config/arm/arm.c (arm_option_override): Use TARGET_HARD_FLOAT_ABI.

From-SVN: r249293