Add riscv64gc-unknown-linux-gnu support

[tblah]

rust support for riscv64gc-unknown-linux-gnu is not yet advertised, but is merged.

There are issues with riscv64gc-unknown-linux-gnu but it works well enough to build many crates (including rustc).

[reitermarkus]

bors try

[bors]

try

Build failed:

• rust-embedded.cross

[tblah]

Thanks for taking a look.

I don't think the failed pipeline was related to this change.
From the logs:

Reading package lists...
W: GPG error: http://archive.debian.org/debian jessie Release: The following signatures were invalid: KEYEXPIRED 1587841717  KEYEXPIRED 1587841717  KEYEXPIRED 1587841717  KEYEXPIRED 1587841717  KEYEXPIRED 1587841717
W: The repository 'http://archive.debian.org/debian jessie Release' is not signed.
+ mkdir -p -m 777 /qemu/powerpc
+ cd /qemu/powerpc
+ apt-get -d --no-install-recommends download busybox:powerpc dropbear:powerpc libc6:powerpc libgcc1:powerpc libstdc++6:powerpc linux-image-4.9.0-0.bpo.6-powerpc:powerpc ncurses-base zlib1g:powerpc
WARNING: The following packages cannot be authenticated!
  dropbear:powerpc
E: Some packages could not be authenticated
The command '/bin/sh -c /linux-image.sh powerpc' returned a non-zero code: 100

[reitermarkus]

@tblah, yeah, the powerpc image seems to keep breaking for various reasons. If you like, please try to fix it. Otherwise feel free to move it into disabled and comment out the line in the CI config.

@Alexhuszagh, pinging you in case you want to have a look since you were the last contributor for the powerpc images.

[tblah]

@reitermarkus it looks to me like the signing keys for the Debian Jessie PowerPC repo have expired so apt is refusing to download dropbear from there. Debian Jessie is still in support but apparently that support doesn't apply to PowerPC. 32-bit PowerPC isn't supported on Debian releases since Jessie.

a7d6482 disables apt signature verification before installing packages from the Debian PowerPC repos. This has obvious security concerns, for example if the Debian package mirror is compromised and made to serve malware. I'll leave this up to you.

[reitermarkus]

bors try

[bors]

try

Build succeeded:

• rust-embedded.cross

[reitermarkus]

bors r+

[bors]

Build succeeded:

• rust-embedded.cross

[reitermarkus]

Thanks, @tblah!