Modify test to expose issue #6492.
Fix opts_str.

opt_val doesn't not fail! for missing options.

Closes #6492
Fix comment.
auto merge of #8135 : dim-an/rust/master, r=pcwalton

Fix std::getopt::opts_str

Closes #6492 (std::getopt::opts_str fails for arguments other than the first one).
Provide a "configure" option to disable LLVM assertions

Builds are considerably faster without assertions, so when working on
e.g. libstd, which doesn't directly interact with LLVM, one might want
to disable them.