RFR(S) : 8219395 : integrate gcov w/ run-test

[Igor Ignatyev]

http://cr.openjdk.java.net/~iignatyev//8219395/webrev.00/index.html
> 65 lines changed: 59 ins; 0 del; 6 mod;

Hi all,

could you please review the patch which makes it easy to run tests on the builds w/ native-code-coverage enabled? to do so the patch
 - sets GCOV_PREFIX env. variable, so .gcda files will be stored in build/*/test-results/gcov-output directory, and makes jtreg to propagate this env. variable to JDK under test
 - adds linux-x64-gcov and macosx-x64-gcov jib profiles
 - changes 'run-test-prebuilt' profile to set GCOV_ENABLED=true if it's the tested profile is -gcov profile

and also fixes comment for JDKOPT_SETUP_CODE_COVERAGE in jdk-options.m4.

webrev: http://cr.openjdk.java.net/~iignatyev//8219395/webrev.00/index.html
JBS: https://bugs.openjdk.java.net/browse/JDK-8219395
testing: 
 - :tier1 on {linux,macos}-x64 and {linux,macos}-x64-gcov
 - checked that *-gcov builds have .gcno files generated and stored in symbols bundle; and regular builds don't 
 - checked that *-gcov runs have .gcda files generated in test-results/gcov-output; and runs on regular builds don't

Thanks,
-- Igor