soft-fp: Add _FP_UNREACHABLE.

This patch makes soft-fp use a new macro _FP_UNREACHABLE in place of
calling abort in unreachable default cases of switch statements.
_FP_UNREACHABLE expands to call __builtin_unreachable for GCC 4.5 and
later; the fallback to abort is thus only for kernel use.

Tested for powerpc-nofpu that installed stripped shared libraries are
unchanged by this patch.  Also tested with the math/ tests for mips64
(in the case of fma there *was* previously an abort call generated,
unlike for the other operations - one switch only deals with a subset
of classes for one operand based on what could have been generated in
the earlier part of fma, whereas the other switches deal with all
combinations of two classes - and this is apparently too complicated
for the default case to have been optimized away).

	* soft-fp/soft-fp.h (_FP_UNREACHABLE): New macro.
	* soft-fp/op-common.h (_FP_MUL): Use _FP_UNREACHABLE instead of
	abort.
	(_FP_FMA): Likewise.
	(_FP_DIV): Likewise.