[cc/coverage] Pass directory to -fprofile-instr-generate

Bug: http://b/157081822

This will be used if LLVM_PROFILE_FILE is not set at runtime.  Some
bionic tests are run without LLVM_PROFILE_FILE set and if they are run
from '/', they fail unable to write to '/default.profraw'.  This may
affect other tests as well.  Setting this to the value defined by
init.rc.

Test: Enable coverage for bionic, ensure relevant bionic-unit-tests
pass.

Change-Id: If1d0e16ce003149fb21d5472d10699ef6582e065