lib: arm: Print test exit status on exit if chr-testdev is not available

The arm64 tests can be run under kvmtool, which doesn't emulate a
chr-testdev device. In preparation for adding run script support for
kvmtool, print the test exit status so the scripts can pick it up and
correctly mark the test as pass or fail.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Change-Id: Ie04fd80370f8007083f30d32bc511a3d0fd2cee4