Correct the type of c_char.

On ARM64, it is supposed to be unsigned char:

  $ aarch64-unknown-cloudabi-cc -dM -E - < /dev/null | grep __CHAR_UNSIGNED__
  #define __CHAR_UNSIGNED__ 1

Author: EdSchouten

src/cloudabi/aarch64.rs

@@ -1,4 +1,4 @@
-pub type c_char = i8;
+pub type c_char = u8;
 pub type c_long = i64;
 pub type c_ulong = u64;
 pub type wchar_t = u32;