Upgrade bunch of EOL docker images to Ubuntu 18.04

[mati865]

When upgrading other Dockerfiles (not modified in this PR) they fail with different errors:
https://travis-ci.com/mati865/libc/jobs/181623075
https://travis-ci.com/mati865/libc/jobs/181623059
https://travis-ci.com/mati865/libc/jobs/181623067

I think it's because of these 2 commits in glibc:
https://sourceware.org/git/?p=glibc.git;a=commit;h=b05cb613e7ae507680c8d914b3e788eea782c0d6
https://sourceware.org/git/?p=glibc.git;a=commit;h=2dba5ce7b8115d6a2789bf279892263621088e74

I have no idea how to fix them so leaving it for somebody who knows what to do.

[rust-highfive]

r? @gnzlbg

(rust_highfive has picked a reviewer for you, use r? to override)

[gnzlbg]

The hard part of the upgrade is probably updating libc-test/build.rs to remove all ignored APIs that are available in 18.04 but that are not available in 17.10. I'd guess that one can do that later.

[mati865]

Probably these 2 could go away once all the images are updated:

• libc/libc-test/build.rs

  Lines 539 to 551 in 8d94e00

  	// These are not in a glibc release yet, only in kernel headers.
  	"AF_XDP"
  	| "PF_XDP"
  	| "SOL_XDP"
  	| "IPV6_FLOWINFO"
  	| "IPV6_FLOWLABEL_MGR"
  	| "IPV6_FLOWINFO_SEND"
  	| "IPV6_FLOWINFO_FLOWLABEL"
  	| "IPV6_FLOWINFO_PRIORITY"
  	if linux =>
  	{
  	true
  	}

• libc/libc-test/build.rs

  Lines 595 to 609 in 8d94e00

  	// There seems to be a small error in EGLIBC's eventfd.h header. The
  	// [underlying system call][1] always takes its first `count`
  	// argument as an `unsigned int`, but [EGLIBC's <sys/eventfd.h>
  	// header][2] declares it to take an `int`. [GLIBC's header][3]
  	// matches the kernel.
  	//
  	// EGLIBC is no longer actively developed, and Debian, the largest
  	// distribution that had been using it, switched back to GLIBC in
  	// April 2015. So effectively all Linux <sys/eventfd.h> headers will
  	// be using `unsigned int` soon.
  	//
  	// [1]: https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/fs/eventfd.c?id=refs/tags/v3.12.51#n397
  	// [2]: http://bazaar.launchpad.net/~ubuntu-branches/ubuntu/trusty/eglibc/trusty/view/head:/sysdeps/unix/sysv/linux/sys/eventfd.h
  	// [3]: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/sys/eventfd.h;h=6295f32e937e779e74318eb9d3bdbe76aef8a8f3;hb=4e42b5b8f89f0e288e68be7ad70f9525aebc2cff#l34
  	"eventfd" if linux => true,

In fact this PR modifies API only for powerpc* because other files that I changed use pinned musl and I found nothing about powerpc in libc-test/build.rs.

[gnzlbg]

@bors: r+

[bors]

📌 Commit 96eb1b2 has been approved by gnzlbg

[gnzlbg]

Thank you!

[bors]

⌛ Testing commit 96eb1b2 with merge 750a7a6...

[bors]

☀️ Test successful - checks-cirrus, checks-travis, status-appveyor
Approved by: gnzlbg
Pushing 750a7a6 to master...