minijail: fix duplicate generated sources rules

The TARGET_ARCH and TARGET_2ND_ARCH sections in Android.mk for
libminijail_generated were both writing to the same files in
$(generated_sources_dir).  Put the files in $(TARGET_ARCH) and
$(TARGET_2ND_ARCH) subdirectories so they don't collide.

Prevents:
./external/minijail/Android.mk:53: warning: overriding commands for target `out/target/product/bullhead/gen/STATIC_LIBRARIES/libminijail_generated_intermediates/libsyscalls.c'
./external/minijail/Android.mk:41: warning: ignoring old commands for target `out/target/product/bullhead/gen/STATIC_LIBRARIES/libminijail_generated_intermediates/libsyscalls.c'
./external/minijail/Android.mk:58: warning: overriding commands for target `out/target/product/bullhead/gen/STATIC_LIBRARIES/libminijail_generated_intermediates/libconstants.c'
./external/minijail/Android.mk:46: warning: ignoring old commands for target `out/target/product/bullhead/gen/STATIC_LIBRARIES/libminijail_generated_intermediates/libconstants.c'

Change-Id: I892bd6998c44f0bce5358d7e71a584284beac02f
1 file changed