soft-fp: Support conditional zero-initialization in declarations.

soft-fp: Support conditional zero-initialization in declarations.

In the Linux kernel, some architectures have a single function that
uses different kinds of unpacking and packing depending on the
instruction being emulated, meaning it is not readily visible to the
compiler that variables from _FP_DECL and _FP_FRAC_DECL_* macros are
only used in cases where they were initialized.  The existing copy of
soft-fp in the Linux kernel uses zero-initialization to avoid warnings
in this case, so while frowned upon as a warning suppression mechanism
in code built for glibc it seems appropriate to have such
zero-initialization conditional on __KERNEL__.  This patch duly adds
it, via a macro _FP_ZERO_INIT that expands to empty for non-kernel
compilations.

Tested for powerpc-nofpu that installed stripped shared libraries are
unchanged by this patch.

	* soft-fp/soft-fp.h (_FP_ZERO_INIT): New macro.  Define depending
	on [__KERNEL__].
	* soft-fp/op-1.h (_FP_FRAC_DECL_1): Use _FP_ZERO_INIT.
	* soft-fp/op-2.h (_FP_FRAC_DECL_2): Likewise.
	* soft-fp/op-common.h (_FP_DECL): Likewise.