soft-fp: Use multiple-include guards.

This patch makes soft-fp headers consistently use multiple-include
guards, something previously done mainly only in the Linux kernel
version.  The guard macros aren't the same as those used in the Linux
kernel, but there seems to be enough variation in such guards in Linux
kernel code that hopefully this version will be acceptable there.

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

	* soft-fp/double.h [SOFT_FP_DOUBLE_H]: New multiple-include guard.
	* soft-fp/extended.h [SOFT_FP_EXTENDED_H]: Likewise.
	* soft-fp/op-1.h [SOFT_FP_OP_1_H]: Likewise.
	* soft-fp/op-2.h [SOFT_FP_OP_2_H]: Likewise.
	* soft-fp/op-4.h [SOFT_FP_OP_4_H]: Likewise.
	* soft-fp/op-8.h [SOFT_FP_OP_8_H]: Likewise.
	* soft-fp/op-common.h [SOFT_FP_OP_COMMON_H]: Likewise.
	* soft-fp/quad.h [SOFT_FP_QUAD_H]: Likewise.
	* soft-fp/single.h [SOFT_FP_SINGLE_H]: Likewise.
	* soft-fp/soft-fp.h (SOFT_FP_H): Define to 1 rather than empty.
	Add comment on closing #endif.