[Patch, AVR]: Better 32=16*16 widening multiplication

[Georg-Johann Lay]

This implements "mulhisi3" and "umulhisi3" widening multiplication
insns if AVR_HAVE_MUL.

I chose the interface as r25:r22 = r19:r18 * r21:r20 which is ok
because only avr-gcc BE will call respective __* support functions in
libgcc.

Tested without regression and hand-tested assembler code.

Johann

	* config/avr/t-avr (LIB1ASMFUNCS): Add _mulhisi3, _umulhisi3,
	_xmulhisi3_exit.
	* config/avr/libgcc.S (_xmulhisi3_exit): New Function.
	(__mulhisi3): Optimize if have MUL*.  Use XJMP instead of rjmp.
	(__umulhisi3): Ditto.
	* config/avr/avr.md (mulhisi3): New insn expender.
	(umulhisi3): New insn expender.
	(*mulhisi3_call): New insn.
	(*umulhisi3_call): New insn.
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