fix(udp): use libc::recvmmsg on Android x86

[mxinden]

On x86-32, socketcall() was historically the only entry point for
the sockets API. However, starting in Linux 4.3, direct system
calls are provided on x86-32 for the sockets API.

https://man7.org/linux/man-pages/man2/socketcall.2.html

Androids x86 even goes as far as automatically dispatching advanced socket calls (e.g. socket or recvmmsg) through socketcall in its Bionic libc:

For example, socket() syscall on i386 actually becomes:
socketcall(__NR_socket, 1, *(rest of args on stack)).

https://android.googlesource.com/platform/bionic/+/refs/tags/android-vts-14.0_r5/libc/SYSCALLS.TXT#19

In addition Android enables seccomp filtering since Android 8:

https://android-developers.googleblog.com/2017/07/seccomp-filter-in-android-o.html

Currently quinn-udp calls recvmmsg directly on Android x86:

quinn/quinn-udp/src/unix.rs

Lines 447 to 448 in c0b1e28

	let ret =
	libc::syscall(libc::SYS_recvmmsg, sockfd, msgvec, vlen, flags, timeout) as libc::c_int;

A direct recvmmsg through libc::syscall(libc::SYS_recvmmsg does not trigger the automatic Android x86 Bionic libc dispatch logic through socketcall, but instead calls the unimplemented recvmmsg syscall. Instead of triggering a libc::ENOSYS, seccomp disallows the call, thus leading to a panic of the application.

This commit changes quinn-udp to use libc::recvmmsg on Android x86, thus leveraging Bionic's automatic dispatch of recvmmsg through socketcall.

Note that this commit only uses libc::recvmmsg on Android x86, not any other Android or Linux variant. Thus quinn-udp would still support missing libc::recvmmsg on those systems. See #1504.

---

Fixes #1947.

Problem itself and fix reproducible on Firefox CI. See e.g. latest CI run startup-x86.

Still unable to reproduce via #1950. I assume this is due to still using a 64bit toolchain when testing x86.

//CC @link2xt following up on #1504, this might be of interest for you.

[djc]

Thanks! If you'd like a release with this, please add a commit that bumps the version number

[Ralith]

This looks identical to the *BSD call above. Should we fold them together?

[mxinden]

Alternative implementation in #1966. Let me know which way forward you prefer.

[mxinden]

Closing here since #1966 is merged and released. Thanks for taking a look!