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.
auto merge of #8147 : dotdash/rust/assertion_option, r=pcwalton

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.