LibF7: sinh: Fix loss of precision due to cancellation for small values.

libgcc/config/avr/libf7/
	* libf7-const.def [F7MOD_sinh_]: Add MiniMax polynomial.
	* libf7.c (f7_sinh): Use it instead of (exp(x) - exp(-x)) / 2
	when |x| < 0.5 to avoid loss of precision due to cancellation.