selftests/bpf: Add config.aarch64

config.aarch64, similarly to config.{s390x,x86_64} is a config enabling
building a kernel on aarch64 to be used in bpf's
selftests/kernel-patches CI.

Signed-off-by: Manu Bretelle <chantr4@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221021210701.728135-3-chantr4@gmail.com