selftests/bpf: Enable kprobe_multi tests for ARM64

The kprobe_multi feature was disabled on ARM64 due to the lack of fprobe
support.

The fprobe rewrite on function_graph has been recently merged and thus
brought support for fprobes on arm64.  This then enables kprobe_multi
support on arm64, and so the corresponding tests can now be run on this
architecture.

Remove the tests depending on kprobe_multi from DENYLIST.aarch64 to
allow those to run in CI. CONFIG_FPROBE is already correctly set in
tools/testing/selftests/bpf/config

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250219-enable_kprobe_multi_tests-v1-1-faeec99240c8@bootlin.com