Give shared modules in unittests the platform-native extension, make PipSqueak a MODULE

As far as I can tell from revision history, there's no good reason to call
these files .so instead of .dll in Windows, so use the normal extension.

Also change PipSquak from SHARED to MODULE -- it's never passed to
target_link_libraries() and only loaded via dlopen(), so MODULE is more
appropriate. This makes it possible to delete a workaround for SHARED ldflags
being not quite right as well.

No intended behavior change.
https://reviews.llvm.org/D46898

llvm-svn: 332487