x86: shrink prefix related disassembler state fields

x86: shrink prefix related disassembler state fields

By changing the values used for "artificial" prefix values,
all_prefixes[] can be shrunk to array of unsigned char. All that
additionally needs adjusting is the printing of possible apparently
standalone prefixes when recovering from longjmp(): Simply check
whether any prefixes were successfully decoded, to avoid converting
opcode bytes matching the "artificial" values to prefix mnemonics.

Similarly by re-arranging the bits assigned to PREFIX_* mask values
we can fit all segment register masks in a byte and hence shrink
active_seg_prefix to unsigned char.

Somewhat similarly with last_*_prefix representing offsets into the
opcode being disassembled, signed char is sufficient to hold all possible
values.