Use attributes for native module ABI and link name

This patch changes how to specify ABI and link name of a native module.

Before:
  native "cdecl" mod llvm = "rustllvm" {...}

After:
  #[abi = "cdecl"]
  #[link_name = "rustllvm"]
  native mod llvm {...}

The old optional syntax for ABI and link name is no longer supported.

Fixes issue #547

src/lib/linux_os.rs

@@ -6,7 +6,9 @@ TODO: Restructure and document
 
 // FIXME Somehow merge stuff duplicated here and macosx_os.rs. Made difficult
 // by https://github.com/graydon/rust/issues#issue/268
-native "cdecl" mod libc = "" {
+#[link_name = ""]
+#[abi = "cdecl"]
+native mod libc {
     fn read(fd: int, buf: *u8, count: uint) -> int;
     fn write(fd: int, buf: *u8, count: uint) -> int;
     fn fread(buf: *u8, size: uint, n: uint, f: libc::FILE) -> uint;
@@ -81,7 +83,8 @@ fn waitpid(pid: int) -> int {
     ret status;
 }
 
-native "cdecl" mod rustrt {
+#[abi = "cdecl"]
+native mod rustrt {
     fn rust_getcwd() -> str;
 }