native: Stop using readdir()

This function is not threadsafe, and is deprecated in favor of the threadsafe
readdir_r variant.

Closes #12692
native: Move from usleep() to nanosleep()

Using nanosleep() allows us to gracefully recover from EINTR because on error it
fills in the second parameter with the remaining time to sleep.

Closes #12689
native: Fix usage of a deallocated mutex

When the timer_helper thread exited, it would attempt to re-acquire the global
task count mutex, but the mutex had previously been deallocated, leading to
undefined behavior of the mutex, and in some cases deadlock.

Another mutex is used to coordinate shutting down the timer helper thread.

Closes #12699
Fix ICE on statics with fancy nullable enums.

Closes #8506.

The `trans::adt` code for statics uses fields with `C_undef` values to
insert alignment padding (because LLVM's own alignment padding isn't
always sufficient for aggregate constants), and assumes that all fields
in the actual Rust value are represented by non-undef LLVM values, to
distinguish them from that padding.

But for nullable pointer enums, if non-null variant has fields other
than the pointer used as the discriminant, they would be set to undef in
the null case, to reflect that they're never accessed.

To avoid the obvious conflict between these two items, the latter undefs
were wrapped in unary LLVM structs to distinguish them from the former
undefs.  Except this doesn't actually work -- LLVM, not unreasonably,
treats the "wrapped undef" as a regular undef.

So this commit just sets all fields to null in the null pointer case of
a nullable pointer enum static, because the other fields don't really
need to be undef in the first place.
std: Move libnative task count bookkeeping to std

When using tasks in Rust, the expectation is that the runtime does not exit
before all tasks have exited. This is enforced in libgreen through the
`SchedPool` type, and it is enforced in libnative through a `bookkeeping` module
and a global count/mutex pair. Unfortunately, this means that a process which
originates with libgreen will not wait for spawned native tasks.

In order to fix this problem, the bookkeeping module was moved from libnative to
libstd so the runtime itself can wait for native tasks to exit. Green tasks do
not manage themselves through this bookkeeping module, but native tasks will
continue to manage themselves through this module.

Closes #12684
auto merge of #12703 : jld/rust/issue-8506, r=alexcrichton

Closes #8506.
rustdoc: Add anchors to section headers

This commit adds a appear-on-over link to all section headers to generated
documentation. Each header also receives an id now, even those generated through
markdown. The purpose of this is to provide easy to link to sections.

This modifies the default header markdown generation because the default id
added looks like "toc_NN" which is difficult to reconcile among all sections (by
default each section gets a "toc_0" id), and it's also not very descriptive of
where you're going.

This chooses to adopt the github-style anchors by taking the contents of the
title and hyphen-separating them (after lower casing).

Closes #12681
debuginfo: Re-introduce the notion of line-table-only debuginfo.
debuginfo: Expose limited debuginfo in command line options
debuginfo: Add test case for limited debuginfo
auto merge of #12687 : alexcrichton/rust/issue-12681, r=huonw

This commit adds a appear-on-over link to all section headers to generated
documentation. Each header also receives an id now, even those generated through
markdown. The purpose of this is to provide easy to link to sections.

This modifies the default header markdown generation because the default id
added looks like "toc_NN" which is difficult to reconcile among all sections (by
default each section gets a "toc_0" id), and it's also not very descriptive of
where you're going.

This chooses to adopt the github-style anchors by taking the contents of the
title and hyphen-separating them (after lower casing).

Closes #12681
auto merge of #12705 : alexcrichton/rust/issue-12692, r=brson

Details are in the commit messages, but this closes a few issues seen with `libnative` recently.
auto merge of #12714 : michaelwoerister/rust/limited-debuginfo, r=alexcrichton

This PR brings back limited debuginfo which allows for nice backtraces and breakpoints, but omits any info about variables and types.

The `-g` and `--debuginfo` command line options have been extended to take an optional argument:
`-g0` means no debug info.
`-g1` means line-tables only.
`-g2` means full debug info.

Specifying `-g` without argument is equivalent to `-g2`.

Fixes #12280