wireguard: selftests: remove ancient kernel compatibility code

Quite a bit of the test suite was designed to work with ancient kernels.
Thankfully we no longer have to deal with this. This commit updates
things that we can finally update and removes things that we can finally
remove, to avoid the build-up of the last several years as a result of
having to support ancient kernels. We can finally rely on suppress_
prefixlength being available. On the build side of things, the no-PIE
hack is no longer required, and we can bump some of the tools, repair
our m68k and i686-kvm support, and get better coverage of the static
branches used in the crypto lib and in udp_tunnel.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>