gnu: rust: Support build with glibc-2.27; update to 1.26.2.

gnu: rust: Support build with glibc-2.27; update to 1.26.2.

* gnu/packages/rust.scm (rust-source): Add "patches" argument.
(rust-bootstrapped-package): Add "patches" argument.
(rust-1.19)[arguments]<#:phases>[patch-tests]: Modify.
[arguments]<#:phases>[patch-aarch64-test]: New phase.
[arguments]<#:phases>[use-readelf-for-tests]: New phase.
[arguments]<#:phases>[remove-unsupported-tests]: New phase.
(rust-1.23): Disable "run-pass/out-of-stack.rs" test.
(rust-1.24)[arguments]<#:phases>[patch-aarch64-test]: Replace.
(rust-1.25)[inputs]: Switch to LLVM 6.0.
(rust-1.25)[arguments]<#:phases>[enable-codegen-tests]: New variable.
(rust): New variable.
* gnu/packages/patches/rust-coresimd-doctest.patch: New file.