Make headless work on aarch64 and armhf (armv7)
Following the two comments at #1854 (comment 44340).
.jar files now get built with
$ ./gradlew --configure-on-demand briar-headless:x86LinuxJar
$ ./gradlew --configure-on-demand briar-headless:aarch64LinuxJar
$ ./gradlew --configure-on-demand briar-headless:armhfLinuxJarRelated to tor-reproducer!13 (merged) and tor-reproducer!14 (closed).
Chat logs at https://chat.briarproject.org/briar/pl/6kgsuccjotg97p7y4xq4tebr6w.
Fixes #1854 (closed).