Set SO_REUSEADDR by default in libnative.

Fixes std::net test error when re-running too quickly.
Add appropriate LLVM module flags for debug info.

Set "Dwarf Version" to 2 on OS X to avoid toolchain incompatibility, and
set "Debug Info Version" to prevent debug info from being stripped from
bitcode.

Fixes #11352.
auto merge of #11864 : comex/rust/11352, r=alexcrichton

Set "Dwarf Version" to 2 on OS X to avoid toolchain incompatibility, and
set "Debug Info Version" to prevent debug info from being stripped from
bitcode.

Fixes #11352.
auto merge of #11845 : xales/rust/libnative, r=alexcrichton

Fixes std::net test error when re-running too quickly.

Suggested by @cmr
Removing do keyword from librustdoc
Removing do keyword from libstd and librustc
Remove do keyword from librustuv
Removing do keyword from libnative
Removing do keyword from librustpkg
Removing do keyword from libextra
Removing do keyword from libgreen
Remove do keyword from test/
Remove do keyword from compiletests
Removing usage of the do keyword from documentation
Removing support for the do syntax from libsyntax and librustc.

Fixes #10815.