Set a BIONIC_COVERAGE .mk variable

Bug: http://b/194128476
Bug: http://b/210012154

Set BIONIC_COVERAGE to true if coverage instrumentation is enabled for
bionic/libc.  This is used in system/core/rootdir/Android.mk to set
the default LLVM_PROFILE_FILE environment.

Test: Run tests with this topic and verify coverage still works and also
test memory-mapped coverage (death tests, JNI code in CTS)

Change-Id: I585f3e43dfcb82dd3c211051d354dd79ee77b1fb