Disable coverage sanitizer when address sanitizer is disabled on libm.

Test: Built bullhead with SANITIZE_TARGET="address coverage".
Change-Id: I9a8f9f42b3b8fdff6c5ff41ddd42a60c04f85a27
diff --git a/libm/Android.bp b/libm/Android.bp
index b1d88d5..7489bfe 100644
--- a/libm/Android.bp
+++ b/libm/Android.bp
@@ -528,6 +528,7 @@
     native_coverage: bionic_coverage,
     sanitize: {
         address: false,
+        coverage: false,
     },
     stl: "none",
 }