Update Ubuntu images to 19.10

[JohnTitor]

Ubuntu 19.04(disco) has already reached EOL, this replaces them with 19.10(eoan).

[rust-highfive]

r? @gnzlbg

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

[qnighy]

19.10(focal)

I guess it's eoan?

[JohnTitor]

I guess it's eoan?

Yeah, you're right!

[JohnTitor]

Notes:

• mips64el-unknown-linux-gnuabi64 and mips-unknown-linux-gnu remain Ubuntu 19.04 since their gcc packages have been gone. Left FIXME comment.
• ignores sys/io.h on *-gnuabihf
• ignores sys/sysctl.h on *-gnu since it has been deprecated
• ignores statx and statx_timestamp structs since somehow headers aren't included correctly. Left FIXME comment.

[JohnTitor]

Actually, it blocks #1638 and CI improvements only, skipping some items though, so going to merge. If anyone finds any fallout, just let me know!