arm64: tolerate host allocation failures

arm64: tolerate host allocation failures

We have no working ARM64 hardware right now.  Stop blocking things.

(Arguably this ought to be done in general for all arches, but
actually bugs in the flight construction code can cause this too, and
we wouldn't want such a regression to pass the osstest
self-push-gate.)

CC: Julien Grall <julien.grall@arm.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
---
v2: Match @@-arm64 jobs (eg build-arm64) too