AVR: ad target/125752 - Simplify double -> 64-bit fixed conversions.

When converting a fixed-point value to double, there is no need to
use ldexp to adjust for FBIT of the result.  Instead, FBIT can be
added to the double exponent without any further ado.  When the
input is +/-0.0 then the outcome of adding FBIT to the exponent
is a small number that __fix[uns]dfdi converts to 0.

	PR target/125752
libgcc/config/avr/libf7/
	* libf7.c (d_to_ufx, d_to_sfx): Remove prototypes.
	(__fractdfda, __fractdfuda, __fractdfta, __fractdfuta)
	(__fractdfdq, __fractdfudq): Move implementation to...
	* libf7-asm.sx: ...here.
	(d_to_fx64): Remove.
	* libf7-common.mk (F7_ASM_PARTS): Add D2dq, D2udq, D2da,
	D2uda, D2ta, D2uta.
	(F7F_asm): Remove d_to_fx64.
	* t-libf7 (LIBF7_DF_CONV): Remove fractdfda, fractdfta,
	fractdfdq, fractdfuda, fractdfuta, fractdfudq.
	* f7-renames.h: Rebuild.

(cherry picked from commit 997a7e79258e7209718c5366b6a1d3be31c76b8b)