RFR 8241285(s): [jdk8u] fail to build hotspot with gcc-8.4.0 with or without COMPILER_WARNINGS_FATAL

[linzang(臧琳)]

Dear All,
     Would you like to help review this tiny fix of build hotspot with gcc-8.4.0?
     Bug: https://bugs.openjdk.java.net/browse/JDK-8241285
     Webrev: http://cr.openjdk.java.net/~lzang/8241285/webrev/

    The fix makes gcc consider COMPILER_WARNINGS_FATAL flag, when it set to false, the warnings from gcc will not be treated as error.

BRs,
Lin