L4Re Target: Add the needed Libraries and locate them

[TobiasSchaffner]

Add the libraries and objects that have to be linked to a get working L4Re Binary using pre- and post-link-args. Additionaly some ld commands had to be passed.

• L4Re libraries and objects will be located by an environment variable.
• gcc libraries and objects will be located using a gcc call.

GCC is mandatory for this target, that might need documentation somewhere. As soon as something mandatory cannot be found, the compiler will panic. This is intended, because the functions involved don't allow the usage of a Result type. libgcc_eh is now passed using -l and crtbeginT.o and crtend.o are now located using gcc -print-filename.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @arielb1 (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[TobiasSchaffner]

As we want the build to fail if the L4RE_LIBDIR environment variable is not set, this behavior is intended.

[00:59:28] 	thread 'target::test_json_encode_decode::x86_64_unknown_l4re_uclibc' panicked at 'Unable to find L4Re library directory: L4RE_LIBDIR not set.', /checkout/src/libcore/option.rs:819:4
[00:59:28] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[00:59:28] 
[00:59:28] 
[00:59:28] failures:
[00:59:28]     target::test_json_encode_decode::x86_64_unknown_l4re_uclibc
[00:59:28] 
[00:59:28] test result: FAILED. 73 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
[00:59:28] 
[00:59:28] error: test failed, to rerun pass '--lib'

Can the test be adjustet or the environment variable be set before running the test?

[arielb1]

r? @alexcrichton - that's more of his area.

[alexcrichton]

This seems fine to me, thanks! The CI test can probably fixed in w/e the same way iOS doesn't cause failures today (which can also fail at runtime when calculating the target). I'll leave it up to you whether you'd like to hardcode all these values, I'm not sure if they change much for y'all!

[alexcrichton]

Hm sorry that's not quite what I had in mind, I believe the target function returns a Result, so if the env var isn't defined can it just return an error?

[TobiasSchaffner]

r? @alexcrichton

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit c60fc4b has been approved by alexcrichton

[bors]

⌛ Testing commit c60fc4b with merge af6298d...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing af6298d to master...