More comments added.
(md_begin): Removed "construct symbols for each register name".
Because register names conflicts with GCC generated function
names.
(avr_operand): Now constant numbers can be used as a register
identifiers (0 as r0, 31 as r31).
(md_assemble): use skip_space () before parsing instruction
operands.
(md_assemble): Handle opcodes with optional operands (lpm,elpm).
(avr_operand): Handle 'a', 'v' and 'z' constraint letters needed
for `fmul', `movw' and `lpm R,Z' instructions.
(avr_operands): Warn if current opcode is a two-word instruction
and previous opcode was cpse/sbic/sbis/sbrc/sbrs.
(avr_opcodes): New commands added.
(REGISTER_P): Check 'a' and 'v' constraint letters.
(mcu_types): New MCU added.
