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
diff --git a/core/base_rules.mk b/core/base_rules.mk
index 9234abe..cebf52b 100644
--- a/core/base_rules.mk
+++ b/core/base_rules.mk
@@ -499,18 +499,22 @@
 # separate the multiple architectures into subdirectories of the testcase folder.
 arch_dir :=
 is_native :=
+multi_arch :=
 ifeq ($(LOCAL_MODULE_CLASS),NATIVE_TESTS)
   is_native := true
+  multi_arch := true
 endif
 ifeq ($(LOCAL_MODULE_CLASS),NATIVE_BENCHMARK)
   is_native := true
+  multi_arch := true
 endif
 ifdef LOCAL_MULTILIB
-  is_native := true
+  multi_arch := true
 endif
-ifdef is_native
+ifdef multi_arch
   arch_dir := /$($(my_prefix)$(LOCAL_2ND_ARCH_VAR_PREFIX)ARCH)
 endif
+multi_arch :=
 
 # The module itself.
 $(foreach suite, $(LOCAL_COMPATIBILITY_SUITE), \