soft-fp: Support floating-point extensions without quieting sNaNs.

soft-fp: Support floating-point extensions without quieting sNaNs.

One special case needed in soft-fp to replace the old version in the
Linux kernel is extending from a narrower floating-point format to a
wider one without quieting signaling NaNs.  (This is for
arch/powerpc/math-emu/lfs.c, where previously it used the old FP_CONV
which didn't do anything special for NaNs, then handled packing
specially for NaNs to avoid quieting at packing time, and discarded
the exceptions from unpacking.)

This patch accordingly refactors FP_EXTEND, creating a separate
_FP_EXTEND_CNAN that offers a choice of how NaNs are handled, with
FP_EXTEND reimplemented as a wrapper that provides the common case of
the IEEE operation that does quiet signaling NaNs and raise exceptions
for them.

Tested for powerpc (e500) that installed stripped shared libraries are
unchanged by this patch.

	* soft-fp/op-common.h (FP_EXTEND): Rename to _FP_EXTEND_CNAN with
	extra argument CHECK_NAN.  Redefine as wrapper around
	_FP_EXTEND_CNAN.