Whoops -- that #include <cxxabi.h> wasn't supposed to sneak in there. If...

Whoops -- that #include <cxxabi.h> wasn't supposed to sneak in there. If present, it may already define a __cxxabiv1::__guard that we could use, but that file comes from libstdc++, and I don't think the NuttX buildroot-based toolchain would have that, which is why we need libxx in the first place.