libquadmath: Provide __BYTE_ORDER, __LITTLE_ENDIAN and __BIG_ENDIAN definitions

My earlier libquadmath change apparently broke mingw32 build, while on Linux
<bits/endian.h> is included and defines these, on Mingw apparently that isn't
the case, while soft-fp wants a guarantee that sfp-machine.h defines these.

2024-04-09  Jakub Jelinek  <jakub@redhat.com>

	* sfp-machine.h (__LITTLE_ENDIAN, __BIG_ENDIAN, __BYTE_ORDER): Define
	if __BYTE_ORDER isn't defined.