Simplify Visual C configuration for -m64/-m32mscoff builds

Ping @rainers 
You should use MODEL in the make file to set the VCBIN_SUBDIR.
Not sure if it can be done though.
Maybe double expansion works.
`VCBIN_SUBDIR_64=bin\x86_64`
`VCBIN_SUBDIR=$(VCBIN_SUBDIR_$(MODEL))`