msvs: Generate proper configurations for mixed platforms

msvs: Generate proper configurations for mixed platforms

Prior to 73c4e284, the generated .sln files didn't contain any
information about the different configurations when using .vcxproj
project files. The MSVS IDE was able to fill this in just fine when
loaded though.

When building for ARM, the obj_int_extract project still is built
for x86, in order for the build process to be able to use
obj_int_extract.exe.

Now that configuration info is generated, it breaks current ARM
setups, since the configurations generated by gen_msvs_sln.sh only
included configurations from the last parsed project file (as
mentioned in the comment).

In these setups, the MSVS IDE generated a third meta-platform, called
"Mixed Platforms". This meta-platform points to either ARM or
Win32 as platform in each of the individual projects.

When the MSVS IDE generated this automatically, it also included
the original ARM and Win32 platforms as separate choices, but these
can be omitted since they don't make sense.

Change-Id: Ie25226496f91af4bb1ad8eb9ae9ca5bfed0433d7