AVR: Tweak lib1funcs-fixed.S::__mulQ64_work.

This patch
- Tweaks the signed overflow handling in __mulQ64_work.
- Rename misnomed fx24-mul.c to fx64-mul-1.c.
- Adds more test cases in fx64-mul-2.c.

libgcc/
	* config/avr/lib1funcs-fixed.S (__mulQ64_work): Tweak
	overflow handling.  Use __negsi2 (non-saturating negation)
	to negate.
gcc/testsuite/
	* gcc.target/avr/fx24-mul.c: Renamed to...
	* gcc.target/avr/fx64-mul-1.c: ...this.
	* gcc.target/avr/fx64-mul-2.c: New test.
	* gcc.target/avr/fx64-mul.h: New file.