cmov: cmov: Incorrect output due to improper zero-extension in aarch64 conditional move operations

A flaw was found in the cmov component of RustCrypto. The aarch64 implementations of Cmov and CmovEq incorrectly assume that high bits are zero-extended when loading values smaller than a register. This can lead to incorrect output when certain high bits are set in a Cmov selector or in the u16 a...