Support Soong native coverage

Soong (or any other prebuilt) can provide a .gcnodir file using
LOCAL_PREBUILT_COVERAGE_ARCHIVE. Just like today, if one isn't provided
for static libraries, an empty one will be created for Make modules to
use.

Also fixes a problem I noticed including whole static libs having
coverage into static libs. It was trying to write to /WHOLE since
PRIVATE_INTERMEDIATES_DIR was not set.

Bug: 32749731
Test: See build/soong change for soong-side tests
Test: Enabled coverage of a soong static lib, then include it in a make
static lib with LOCAL_WHOLE_STATIC_LIBRARIES and ensure that the any
shared libraries using the make static lib get the embedded coverage
information.
Change-Id: I32762f099c9757074fec922ee4822f819c9ceaf5
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index bae38c5..614a7ca 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -164,6 +164,7 @@
 LOCAL_PICKUP_FILES:=
 LOCAL_POST_INSTALL_CMD:=
 LOCAL_POST_LINK_CMD:=
+LOCAL_PREBUILT_COVERAGE_ARCHIVE:=
 LOCAL_PREBUILT_EXECUTABLES:=
 LOCAL_PREBUILT_JAVA_LIBRARIES:=
 LOCAL_PREBUILT_JNI_LIBS:=