Correct the type of c_char.
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

rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang/libc,rust-lang/libc,rust-lang-nursery/libc