Remove Freeze bounds from sync::MutexArc

With Rc no longer trying to statically prevent cycles (and thus no
longer using the Freeze bound), it seems appropriate to remove that
restriction from MutexArc as well.
Remove the compile-fail test that's now obsolete
backticks for syntax elements
Fix a segfault in the rustuv tests

The details can be found in the comments I added to the test, but the gist of it
is that capturing output injects rescheduling a green task on failure, which
wasn't desired for the test in question.

cc #12340
Test ordering of glue_drop and glue_takew in self-re-assignment. Closes #3290.
Made fail_bounds_check more careful with strings.

Fixes GH #11976.
Update compiler-rt

Closes #12356
auto merge of #12351 : kud1ing/rust/backticks, r=alexcrichton
auto merge of #12354 : alexcrichton/rust/fix-rustuv-segfault, r=cmr

The details can be found in the comments I added to the test, but the gist of it
is that capturing output injects rescheduling a green task on failure, which
wasn't desired for the test in question.

cc #12340
auto merge of #12330 : nick29581/rust/glue, r=huonw

closes #3290
auto merge of #12336 : kballard/rust/mutexarc-no-freeze, r=alexcrichton

With Rc no longer trying to statically prevent cycles (and thus no
longer using the Freeze bound), it seems appropriate to remove that
restriction from MutexArc as well.

Closes #9251.
auto merge of #12357 : chromatic/rust/gh_11976_fail_bounds_check_str, r=alexcrichton

Fixes #11976.
auto merge of #12359 : alexcrichton/rust/update-compiler-rt, r=luqmana

Closes #12356
move extra::test to libtest