Fix argv.rs under the LLVM compiler.

The call to rust_start was assuming that all rust main() functions have the
same signature, but the compiler doesn't actually canonicalize them.  So
instead just match the C signature of rust_start, and cast.

src/Makefile

@@ -386,7 +386,6 @@ TEST_XFAILS_LLVM := $(addprefix test/run-pass/, \
                       acyclic-unwind.rs \
                       alt-pattern-simple.rs \
                       alt-tag.rs \
-                      argv.rs \
                       basic.rs \
                       bind-obj-ctor.rs \
                       bind-thunk.rs \