AVR: target 113927 - Simple code triggers stack frame for Reduced Tiny.

The -mmcu=avrtiny cores have no ADIW and SBIW instructions.  This was
implemented by clearing all regs out of regclass ADDW_REGS so that
constraint "w" never matched.  This corrupted the subset relations of
the register classes as they appear in enum reg_class.

This patch keeps ADDW_REGS like for all other cores, i.e. it contains
R24...R31.  Instead of tests like  test_hard_reg_class (ADDW_REGS, *)
the code now uses  avr_adiw_reg_p (*).  And all insns with constraint "w"
get "isa" insn attribute value of "adiw".

Plus, a new built-in macro __AVR_HAVE_ADIW__ is provided, which is more
specific than __AVR_TINY__.

gcc/
	PR target/113927
	* config/avr/avr.h (AVR_HAVE_ADIW): New macro.
	* config/avr/avr-protos.h (avr_adiw_reg_p): New proto.
	* config/avr/avr.cc (avr_adiw_reg_p): New function.
	(avr_conditional_register_usage) [AVR_TINY]: Don't clear ADDW_REGS.
	Replace test_hard_reg_class (ADDW_REGS, ...) with calls to
	* config/avr/avr.md: Same.
	(attr "isa") <tiny, no_tiny>: Remove.
	<adiw, no_adiw>: Add.
	(define_insn, define_insn_and_split): When an alternative has
	constraint "w", then set attribute "isa" to "adiw".
	* config/avr/avr-c.cc (avr_cpu_cpp_builtins) [AVR_HAVE_ADIW]:
	Built-in define __AVR_HAVE_ADIW__.
	* doc/invoke.texi (AVR Options): Document it.