Current version of qemu in ChromeOS does not build with libc++. A newer upstream version can be built with libc++ but has some issues related to static build.
According to vapier@ in https://bugs.chromium.org/p/chromium/issues/detail?id=747613#c3:
iirc, upstream has messed around with the static settings on the binaries, and i haven't gotten around to fixing it. so you end up with non-static qemu-system programs and they don't work outside of the chroot.
Comment 1 by cmt...@chromium.org
, Feb 5 2018