qemu: Make qemu buildable inside qemux86/qemuppc target

qemu: Make qemu buildable inside qemux86/qemuppc target

It requires libx11 and libGL(if enabled GL) for building, which introduce extra
dependence in qemuppc target. Futhermore, qemu's GL code is x86-oriented, thus
some hacks needed for building in qemupcc. For simplicity, just make qemu GL's
code for native only, so that qemu building is okay in world build.

[YOCTO #1011] got fixed.

Signed-off-by: Zhai Edwin <edwin.zhai@intel.com>