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.

src/comp/driver/rustc.rs

@@ -477,7 +477,7 @@ fn build_output_filenames(ifile: str, ofile: option::t<str>,
               link::output_type_none. { "none" }
               link::output_type_bitcode. { "bc" }
               link::output_type_assembly. { "s" }
-              link::output_type_llvm_assembly. { "s" }
+              link::output_type_llvm_assembly. { "ll" }
               // Object and exe output both use the '.o' extension here
               link::output_type_object. | link::output_type_exe. {
                 "o"