Various fixes for x86_64 on linux

Configure LLVM correctly, use the right data layout, add the readlink
function back, fix C constants, etc.

src/lib/linux_os.rs

@@ -50,6 +50,7 @@ native mod libc {
     fn unsetenv(n: str::sbuf) -> c_int;
     fn pipe(buf: *mutable fd_t) -> c_int;
     fn waitpid(pid: pid_t, &status: c_int, options: c_int) -> pid_t;
+    fn readlink(path: str::sbuf, buf: str::sbuf, bufsize: size_t) -> ssize_t;
 }
 
 mod libc_constants {