Name atan2 arguments consistently

Doing `methods(atan2)` reveals an inconsistency in the names of the arguments x and y. The documentation about atan2 says `atan2(y, x)`, but the names in the functions are inverted.

backport of c5d868f1832381d41dd14ad2efe3c706d1b7dc69

Changed argument names for atan2 in float16.jl

backport of 0ed59050d6cb2cb489eed004305a8a759cb36cfb