configure: arrange to use appropriate objcopy

configure: arrange to use appropriate objcopy

Using the system objcopy is wrong when other configure checks have
probed a different set of binutils (I've noticed the problem on a system
where the base objcopy can't deal with compressed debug sections).
Arrange for the matching one to be picked up, first and foremost if an
"in tree" one is available, by mirroring respective logic already
present for nm.

gcc/

	* Makefile.in (ORIGINAL_OBJCOPY_FOR_TARGET): New.
	* configure.ac: Check for objcopy, producing
	ORIGINAL_OBJCOPY_FOR_TARGET.
	* configure: Update accordingly.
	* exec-tool.in (ORIGINAL_OBJCOPY_FOR_TARGET): New.
	Handle objcopy.