Fix for threads on high end Windows systems

[aachrisg]

Due to LibUV lacking the necessary pieces, threaded Julia code is only able to take advantage of 1/2 to 1/8 of the available CPU power on Windows systems that either have >64 HW threads (such as the 64-core AMD threadripper), or have more than one CPU socket (such as common Xeon systems).

This fork of LibUV https://github.com/aachrisg/libuv (I submitted a pull request) contains the necessary code in src/win/thread.c to work around this issue. Without this code, my workstation will only show 50% cpu usage now matter how many threads are created.

It is setup to have no affect on systems that wouldn't have the problem.

[vchuravy]

x-ref: libuv/libuv#3458