[cc/coverage] Define a macro during coverage builds

Bug: http://b/186576313

During a coverage build, pass the __ANDROID_CLANG_COVERAGE__ macro to
native compilations.  This allows tools like dalvikvm and dex2oat, which
would otherwise call _exit(), to write profiles on coverage builds by
calling exit() instead.

Test: `m CLANG_COVERAGE=true NATIVE_COVERAGE_PATHS="art" nothing`
          and check compiler flags in out/soong/build.ninja.
Change-Id: I18315b89170abdd650e4c6c55577688348c42225
(cherry picked from commit 551b06d9)