Fix bionic_coverage for kati.

Kati won't override something set with :=, only ?=.

Note that the actually reveals that our coverage build has been broken
for... some time. The issue is that we have static libraries built
with clang and others built with gcc linked into the same .so. If we
use both libclang_rt.profile and libgcov together, we get duplicate
symbol errors. If we only use one or the other, we don't have all the
symbols we need.

This will be fixed once chh's TLS stuff makes it into our clang and we
can start building libc_thread_atexit_impl with clang.

Change-Id: I45d0f2e558a881ad8be71088102320724ced8fe5

libc/Android.mk

@@ -1,6 +1,6 @@
 LOCAL_PATH := $(call my-dir)
 
-bionic_coverage := false
+bionic_coverage ?= false
 
 # Make everything depend on any changes to included makefiles.
 libc_common_additional_dependencies := $(LOCAL_PATH)/Android.mk