base_rules.mk: Don't use is_native for MULTILIB modules.

Currently we determine if a test artifact has multiple build
outputs by checking if it is NATIVE_TEST, NATIVE_BENCHMARK, or
LOCAL_MULTILIB and declare the module is_native.

This is technically not true therefore let's use a different
variable to determine if we need multiple architecture directories.

Bug: 71554249
Test: make sl4a -j ; Results in an autogenerated empty configuration file.
Change-Id: I17c8f3ef7e1cd502b57e25c71688f12a35a9787d