[RFR] JDK-8156980: Hotspot build doesn't have -std=gnu++98 gcc option

[Martin Buchholz]

We seem to have some confusion about flags for C vs. flags for C++.  Most
flags for most toolchains apply to both C and C++, so it's understandable
that we want to unify them.  But some flags, notably -std, are
language-specific.  We have both EXTRA_CFLAGS and EXTRA_CXXFLAGS, so we
should expect EXTRA_CFLAGS to only apply to C.