Fix inline asm constraints of wdt_enable, wdt_disable.

[sprintersb]

This patchlet fixes inline assembly constraints of wdt_enable and wdt_disable from include/avr/wdt.h.

Some of the operands like uint8_t temp are not inputs but are being assigned to in inline asm, so they have to be moved to the ouput operands.

If a register uses cbr or sbr, then it must be in reg-class upper registers d, not general regs r.

One #define needed wrapping into do-while (or removal of terminal ;).

[sprintersb]

Adjusting constraints might be more comprehensible than masking like proposed by #888.

[sprintersb]

Implemented as 4540e09