Update MinGW to default to better exceptions

sjlj is an old style of exception handling that's slower. It also has a
problem linking against the Ruby DLL. Change the default to dwarf/seh
depending on architecture.

Also add params so the exception handling and thread library can be
selected when installing MinGW.
Except for here with `-x86`. It would install the x86 version into mingw64 folder. I don't think you can detect that it was passed either. :( 

Any reason to have different folders?