Add global GCOV_PREFIX option.

When native coverage is enabled, add a global GCOV_PREFIX
environment variable specifying that gcda files be output
with path prefix /data/local/tmp.

Bug: 35635587
Test: make NATIVE_COVERAGE=true; check init.environ.rc
Change-Id: I40972aea3ca3168d0687bdc93e9d4b7b3a1071b9