AVR Linker: Allow the start of the data region to be specified on the linker command line.  [Fix PR number in ChangeLog entry]

	PR 29741
	* scripttempl/avr.sc (__DATA_REGION_ORIGIN__): Define.  If a value
	has not been provided on the command line then use DATA_ORIGIN.
	(MEMORY): Use __DATA_REGION_ORIGIN__ as the start of the data region.