Update build to optionally configure with nmake

Update build to optionally configure with nmake

For formatting, we want to be able to obtain the compile_commands.json
file so that we do not have to 1) do a full build of coreclr and 2)
generate it ourselves from the build log. This change modifies build.cmd
to take an option "usenmakemakefiles." This option will set
__NMakeMakefiles to 1, which is used by the gen-buildsys-win.bat script.
This option also sets all the same options as configureonly, as we cannot
do a full build using NMake Makefiles, and we only want to use this to
force CMake to generate a compile_commands.json file for the jit
directory, which it can do using NMake Makefiles, but not Visual Studio.
The update to gen-buildsys-win.bat checks the value of __NMakeMakefiles,
and if it is set to 1, it sets the CMake Generator to NMake Makefiles.