android: Use system = 'android' in crossfile

Fixes the following libarchive error when building with the libarchive wrap subproject:

In file included from ../subprojects/libarchive-3.7.2/libarchive/archive_write_open_memory.c:33: ../subprojects/libarchive-3.7.2/libarchive/archive.h:101:10: fatal error: 'android_lf.h' file not found

This also removes the version suffixes from the .so's that are referenced in the mesa3d_cross.mk file as those are not present on the Android target.

Signed-off-by: Joshua Ashton joshua@froggi.es