Figure out / fix X255 failures

[nickray]

salty/qemu-tests/src/bin/x25519_on_target/main.rs

Line 18 in b6322d8

const EXEMPTED_FAILURES: &'static [u32] = &[128, 141, 151];

[enrikb]

I found a first hint pointing to field element subtraction: enrikb@d74b5ab