Support "z" output constraint

[4ntoine]

/Applications/A.app/Contents/Resources/Java/hardware/a/cores/a/wiring_digital.c:176:7: error: 
      invalid output constraint '=z' in asm
        if (*portInputRegister(port) & bit) return HIGH;
             ^
/Applications/A.app/Contents/Resources/Java/hardware/a/cores/a/A.h:146:54: note: expanded
      from macro 'portInputRegister'
#define portInputRegister(P) ( (volatile uint8_t *)( pgm_read_word( port_to_input_PGM + (P))) )
                                                     ^
/Applications/A.app/Contents/Resources/Java/hardware/tools/avr/avr/include/avr/pgmspace.h:705:41: note: 
      expanded from macro 'pgm_read_word'
#define pgm_read_word(address_short)    pgm_read_word_near(address_short)
                                        ^
/Applications/A.app/Contents/Resources/Java/hardware/tools/avr/avr/include/avr/pgmspace.h:418:43: note: 
      expanded from macro 'pgm_read_word_near'
#define pgm_read_word_near(address_short) __LPM_word((uint16_t)(address_short))
                                          ^
/Applications/A.app/Contents/Resources/Java/hardware/tools/avr/avr/include/avr/pgmspace.h:399:29: note: 
      expanded from macro '__LPM_word'
#define __LPM_word(addr)    __LPM_word_classic__(addr)
                            ^
/Applications/A.app/Contents/Resources/Java/hardware/tools/avr/avr/include/avr/pgmspace.h:291:28: note: 
      expanded from macro '__LPM_word_classic__'
        : "=r" (__result), "=z" (__addr16)  \
                           ^

code:

#define __LPM_word_classic__(addr)          \
(__extension__({                            \
    uint16_t __addr16 = (uint16_t)(addr);   \
    uint16_t __result;                      \
    __asm__                                 \
    (                                       \
        "lpm"           "\n\t"              \
        "mov %A0, r0"   "\n\t"              \
        "adiw r30, 1"   "\n\t"              \
        "lpm"           "\n\t"              \
        "mov %B0, r0"   "\n\t"              \
        : "=r" (__result), "=z" (__addr16)  \
        : "1" (__addr16)                    \
        : "r0"                              \
    );                                      \
    __result;                               \
}))

[dylanmckay]

Closing because the issue is a symptom of issue #23