z80: Format hex numbers with leading zero if necessary.

Assembler syntax requires that all numbers start with a decimal
digit. Introduce a custom 'H' format for prefixing a leading 0
to hexadecimal numbers that would otherwise start with a letter.
z80: Output jump offsets relative to instruction start.

Most assemblers recognize the symbol $ for the address of the
current instruction. Make use of this to output relative jump
instructions using the $[+-]offset syntax, with offset being
the displacement minus the instruction length.