Support for cygwin64

[mcaldi]

Refered to: https://sourceforge.net/p/poderosa/discussion/455998/thread/617488270b/

When starting a cygwin session an error is displayed:
"The code execution cannot proceed beacause cyggcc_s-1.dll was not found. Reinstalling the program may fix this problem".

Workaround:
Copy this directory: https://osdn.net/projects/ttssh2/scm/svn/tree/head/tags/teraterm-4_106/cygterm/
and cygterm.exe (from cygterm+-x86_64 subfolder) under {poderosa_installation_path}\Protocols\cygterm