translate / export weak lang items

[japaric]

see #51671 for details

fixes #51671
fixes #51342

r? @michaelwoerister or @alexcrichton

[rust-highfive]

The job x86_64-gnu-llvm-3.9 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

[00:04:47] travis_fold:start:tidy
travis_time:start:tidy
tidy check
[00:04:48] tidy error: /checkout/src/test/run-make-fulldeps/issue-51671/app.rs: incorrect license
[00:04:49] some tidy checks failed
[00:04:49] 
[00:04:49] 
[00:04:49] command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0-tools-bin/tidy" "/checkout/src" "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "--no-vendor" "--quiet"
[00:04:49] 
[00:04:49] 
[00:04:49] failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test src/tools/tidy
[00:04:49] Build completed unsuccessfully in 0:01:44
[00:04:49] Build completed unsuccessfully in 0:01:44
[00:04:49] Makefile:79: recipe for target 'tidy' failed
[00:04:49] make: *** [tidy] Error 1

The command "stamp sh -x -c "$RUN_SCRIPT"" exited with 2.
travis_time:start:1eed1000
$ date && (curl -fs --head https://google.com | grep ^Date: | sed 's/Date: //g' || true)
---
travis_time:end:24f472dd:start=1529604655958727064,finish=1529604655966101255,duration=7374191
travis_fold:end:after_failure.3
travis_fold:start:after_failure.4
travis_time:start:16ee8967
$ head -30 ./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers || true
head: cannot open ‘./obj/build/x86_64-unknown-linux-gnu/native/asan/build/lib/asan/clang_rt.asan-dynamic-i386.vers’ for reading: No such file or directory
travis_fold:end:after_failure.4
travis_fold:start:after_failure.5
travis_time:start:07260aeb
$ dmesg | grep -i kill

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[alexcrichton]

@bors: r+

[bors]

📌 Commit cce7007 has been approved by alexcrichton

[bors]

⌛ Testing commit cce7007 with merge e3bf634...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing e3bf634 to master...