Do not enable coverage for platform/llndk stub libraries

Bug: http://b/116873221

These are used only for linking, and secondly, don't link with libc or
other system libraries.

Test: Build system/core/libnativeloader with coverage
Change-Id: I37eae572a5eaa00f0e8c72bc1453e967b5bcb577
diff --git a/cc/coverage.go b/cc/coverage.go
index 79f7d7d..cf67c9f 100644
--- a/cc/coverage.go
+++ b/cc/coverage.go
@@ -111,6 +111,8 @@
 			// Just turn off for now.
 		} else if c.useVndk() || c.hasVendorVariant() {
 			// Do not enable coverage for VNDK libraries
+		} else if c.IsStubs() {
+			// Do not enable coverage for platform stub libraries
 		} else if c.isNDKStubLibrary() {
 			// Do not enable coverage for NDK stub libraries
 		} else if c.coverage != nil {