auto merge of #10874 : vadimcn/rust/integrated-as, r=alexcrichton

Last LLVM update seems to have fixed whatever prevented LLVM integrated assembler from generating correct unwind tables on Windows.   This PR switches Windows builds to use internal assembler by default.
Compilation via external assembler can still be requested via the newly added `-Z no-integrated-as` option.

Closes #8809

diff --cc src/librustc/driver/driver.rs
Simple merge