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.
Changed argument names for atan2 in float16.jl
Merge pull request #8940 from gasagna/patch-1

Name atan2 arguments consistently