libgcc: Enable XF mode conversions to/from DFP modes only if supported

Some targets do not support XF mode (eg AArch64), so don't build the
corresponding to/from DFP modes convertion routines if
__LIBGCC_HAS_XF_MODE__ is not defined.

2022-03-31  Christophe Lyon  <christophe.lyon@arm.com>

	libgcc/config/libbid/
	* _dd_to_xf.c: Check __LIBGCC_HAS_XF_MODE__.
	* _sd_to_xf.c: Likewise.
	* _td_to_xf.c: Likewise.
	* _xf_to_dd.c: Likewise.
	* _xf_to_sd.c: Likewise.
	* _xf_to_td.c: Likewise.