AVR: target/114981 - Tweak __builtin_powif / __powisf2

Implement __powisf2 in assembly.

	PR target/114981
libgcc/
	* config/avr/t-avr (LIB2FUNCS_EXCLUDE): Add _powisf2.
	(LIB1ASMFUNCS) [!avrtiny]: Add _powif.
	* config/avr/lib1funcs.S (mov4): New .macro.
	(L_powif, __powisf2) [!avrtiny]: New module and function.

gcc/testsuite/
	* gcc.target/avr/pr114981-powif.c: New test.