x86_64-linux: lower glibc version requirement to 2.17 (compatible with centos 7)

[japaric]

This PR lowers the glibc version requirement of the x86_64-linux-gnu binaries produced by cross to v2.17 (from v2.23). With this change x86_64-linux applications build with cross can run on LTS distributions like centos 7.

The implementation consists of changing the base image of the x86_64-gnu image from ubuntu:16.04 to centos:7
The existing scripts were adapted as follows:

• package management has been refactored into install_packages and purge_packages functions that work under ubuntu and centos
• added if_centos and if_ubuntu helper functions to handle steps specific to either distribution
• qemu.sh: centos doesn't provide libattr, libcap and libpixman as static libraries so those libraries have to be compiled when the script runs on centos
• linux-image.sh: this script is very ubuntu/debian specific as it relies on apt-get to build a minimal debian sysroot. Instead of modifying the script to make it work on centos, I opted for running the script in a fresh ubuntu:16.04 docker container and copying the output debian sysroot (/qemu) into the centos based image

[japaric]

note that due #499 the qemu-system runner is not being tested in CI. To ensure this PR is not breaking qemu-system on x86_64, PR #500 should land first.

[japaric]

https://dev.azure.com/rust-embedded/cross/_build/results?buildId=791&view=logs&j=14306e81-fbe4-59fd-bc5f-032f2af5fc08&t=883603b3-149d-599e-2f7b-6080333ed52c&l=49

$ cargo install --path . --force
(..)
$ cross build --target x86_64-unknown-linux-gnu --release
(..)
/target/release/build/cross-dbf0d01025c300c1/build-script-build: /lib64/libc.so.6: version `GLIBC_2.18' not found (required by /target/release/build/cross-dbf0d01025c300c1/build-script-build)

interesting the first cargo install command, which runs outside docker, creates artifacts that depend on glibc 2.18+ (CI env is ubuntu 20.04), then the second cross command reuses and runs those artifacts (the build scripts) which fails because there's only a glibc 2.17 inside the docker. a cargo clean should fix the issue (you should not mix cargo build --target $T and cross build --target $T commands; that's a known issue/limitation)

(I was under the impression that cargo install was the same as cargo build, which confusingly is not the same as cargo build --target x86_64-unknown-linux-gnu (where target = host) because they use different output target directories (target/debug vs target/x86_64-unknown-linux-gnu/debug. maybe some subtle semantic changed in one of those commands at some point -- or I may be misremembering things)

[japaric]

https://dev.azure.com/rust-embedded/cross/_build/results?buildId=792&view=logs&j=8f8c9133-e1c4-5d42-4562-da6ba7afb96e&t=3c0ce6f7-5e36-5685-3cbb-bc4b1fce52ae&l=16436

curl --retry 3 -sSfL https://pkg.freebsd.org/FreeBSD:12:i386/quarterly/All/openssl-1.1.1h,1.txz -O
curl: (22) The requested URL returned error: 404 Not Found

that has failed twice in a row so the URL is probably gone; let's figure out which new URL to use ...

[japaric]

PR #502 should fix the freebsd failure

[japaric]

Now that #502 has landed, CI for this PR should be green. Could I get some feedback on this PR? (@reitermarkus maybe?)

[reitermarkus]

bors try

[bors]

try

Build failed:

• rust-embedded.cross

[japaric]

(the "try" build failed due to a network error but the 'multi-stage' commit is green on CI)

[reitermarkus]

bors r+

[bors]

Build failed:

• rust-embedded.cross

[reitermarkus]

bors retry

[bors]

Build failed:

• rust-embedded.cross

[japaric]

it's always the musl.sh script that's failing due to "curl: (22) The requested URL returned error: 502 Bad Gateway". I cannot repro locally but the CI fails with a different target each time. Let's try one more time?

bors retry

[bors]

Build succeeded:

• rust-embedded.cross
• shellcheck