RFR(XXS): 8047321: Hotspot debug builds with clang are broken

[Igor Veresov]

JDK-8032045 specifies -Og optimization option for debug builds that is not supported by clang. 
The fix is to keep it being -O0 for clang.

Webrev: http://cr.openjdk.java.net/~iveresov/8047321/webrev.00/

Thanks,
igor