Use ".ll" as default suffix of LLVM assembly file

Use ".ll" as default suffix of LLVM assembly file

This commit is a follow up of Issue #1147.

Althought there are some inconsistency about this naming convention in
LLVM. For example, `clang' write LLVM assembly to a file with ".s" suffix,
while both `llvm-dis' and `opt' write to files with ".ll" suffices. We
think ".ll" makes more sense.

Also rustc manual page is updated.