AVR: target/114981 - Support __builtin_powi[l] / __powidf2.

This supports __powidf2 by means of a double wrapper for already
existing f7_powi (renamed to __f7_powi by f7-renames.h).
It tweaks the implementation so that it does not perform trivial
multiplications with 1.0 any more, but instead uses a move.
It also fixes the last statement of f7_powi, which was wrong.
Notice that f7_powi was unused until now.

	PR target/114981
libgcc/config/avr/libf7/
	* libf7-common.mk (F7_ASM_PARTS): Add D_powi
	* libf7-asm.sx (F7MOD_D_powi_, __powidf2): New module and function.
	* libf7.c (f7_powi): Fix last (wrong) statement.
	Tweak trivial multiplications with 1.0.

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