Use correct flag on big endian machines.

This will fix `tests::sign` test case on mips/mips64. Verified with
`cross`[1] tool.

    cross test --target mips-unknown-linux-gnu
    cross test --target mips64el-unknown-linux-gnuabi64

Unfortunately this fix doesn't make `tests::test_debug_output` test case
pass, but this is about debug output so its not as critical as this
patch. See #63 for a discussion.

[1]: https://github.com/japaric/rust-cross