Fix underflow generation in soft-fp.

Fix underflow generation in soft-fp.

	* soft-fp/soft-fp.h (FP_CUR_EXCEPTIONS): Define.
	(FP_TRAPPING_EXCEPTIONS): Provide default implementation.
	* sysdeps/sparc/sparc32/soft-fp/sfp-machine.h
	(FP_TRAPPING_EXCEPTIONS): Define.
	* sysdeps/sparc/sparc64/soft-fp/sfp-machine.h
	(FP_TRAPPING_EXCEPTIONS): Define.
	* soft-fp/op-common.h (_FP_PACK_SEMIRAW): Signal underflow for
	subnormals only when inexact has been signalled or underflow
	exceptions are enabled.
	(_FP_PACK_CANONICAL): Likewise.