The cross-built CI runs using GiotHub actions were using QEMU to run `make
check`. Instead, the host should run `make check` and the actual test binary
should be run under QEMU.
This change configures the cross build to use a (new) qemu-test-driver script as
the test harness, which in turn invokes the test under QEMU. The tests are
linked to the libunwind libraries statically to avoid complicating the search
paths and invoking libtool.
Fixes#401.
