8156980: Hotspot build doesn't have -std=gnu++98 gcc option 8157358: Syntax error in TOOLCHAIN_CHECK_COMPILER_VERSION Summary: Add flags for GCC 6 to JVM_CFLAGS and fix prefix handling Reviewed-by: erikj, kbarrett

^build/
^dist/
^.idea/
nbproject/private/
^webrev
^.hgtip
^.bridge2
^.jib/
.DS_Store
.metadata/
.recommenders/