z80: fix compatibility with big-endian

[rofl0r]

the macros used are exposed by default on gcc >= 4.8.x, and clang 3.1 and most other compilers.

before: $ qemu-ppc z80test zexall.cim *** TEST: zexall.cim

*** 25390418 instructions executed on 25390418 cycles (expected=268500992, diff=46734978649)

after: $ qemu-ppc z80test zexall.cim *** TEST: zexall.cim Z80all instruction exerciser <adc,sbc> hl,<bc,de,hl,sp>.... OK add hl,<bc,de,hl,sp>.......... OK ...