Set CXX to a nonexistent compiler when building glibc

We set CXX=riscv64-unknown-linux-gnu-g++ (or something like that) when
building glibc, but don't actually build a C++ compiler during stage 1.
Under normal conditions this shouldn't cause any issues: that's either a
working compiler (from PATH) or doesn't exist, in which case glibc's
build scripts do the right thing and turn off C++.  Unfortunately I'm in
the pathological situation of having a slightly-incompatible g++ in
PATH, which glibc then attempts to use only to throw an error when
linking (glibc isn't testing that much of the C++ toolchain, see
<https://sourceware.org/bugzilla/show_bug.cgi?id=24183

>).

This simply sets CXX to something that's unlikely to exist during the
glibc build, so the build scripts don't get confused.  I tried setting
this to false, but everything was italic.

Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>