Select correct %PROGRAMFILES% based on Win32 vs Win64 in sh.cmd

Msysgit does not yet have an official 64 bit build, so any msysgit installation on a 64bit system ends up in the Program Files (x86) folder.

The sh.cmd in the windows make/winx86/cmd blindly sets a variable for MSYSGIT to the %PROGRAMFILES% directory, which on a 64 bit system is incorrect.

This PR puts a conditional if around the MSYSGIT definition and uses either %PROGRAMFILES% or %PROGRAMFILES(x86)% dependent upon architecture.

For me (at least) this allows a cmd.exe to be launched, and the sh.cmd script to find git's bash.exe. Previously it would not.

I've also removed the exit call, and just set the script to goto :eof, so at least the error/echo'ed messages can be seen before the shell disappears.