No UD_ATTRIBUTE_PACKED for ud_mnemonic_code

I don't think that packing the enum into 16-bit is advantageous in any way, also it doesn't happen on MSVC, for which the macro is defined to be empty. The default of using a 32-bit type for the enum shouldn't change the ud struct's size (at least it doesn't on Clang/GCC for 64-bit), only the offset of the mnemonic member would be aligned to 4 bytes.