Add support to nexttoward{,f,l}

[Johan Oudinet]

Hi,

I've encountered a wine error when running an application, saying that
a function is not implemented:
wine: Call from 0x7b43ce3c to unimplemented function
api-ms-win-crt-math-l1-1-0.dll.nexttowardf, aborting

I've looked into the wine's git repository and found that this
function is declared as a stub, while the equivalent nextafter
variants have been defined since
072f34be730761faa2fabefa8e4a3306d19c4a63

I'm new to wine development and know nothing about windows but I think
I should be able to enable nexttoward, nexttowardf, and nexttowardl
functions, just by following the modifications in the commit above.

According to cppreference, such functions have been added at the same time:
http://en.cppreference.com/w/cpp/numeric/math/nextafter

I'd like your opinion before I submit a patch here.

Best,
-- 
Johan