Use $(my_prefix)OS instead of HOST_CROSS_OS

The latter is not module-specific and affects all host modules.  Also
remove the check for LOCAL_IS_HOST_MODULE and use  $(my_prefix)OS.

This should be future-proof, where may use different configurations of
cross-compilations across Linux, Darwin, Windows.

Test: m native
Change-Id: I0a6255a8b83960ffb543b593d4a50d963be1de88