gas/avr: Prevent incorrect overflow errors for diff fixups.

gas/avr: Prevent incorrect overflow errors for diff fixups.

When fixups are converted to a difference type within md_apply_fix, we
previously left the contents of VALP (the value that was initially
computed within write.c:fixup_segment) unchanged.  This is harmless,
except that this value is used within write.c:fixup_segment once we
return from md_apply_fix to perform an overflow check.

In some cases, the value computed in write.c:fixup_segment is so wrong
that an overflow error can be triggered.  These errors are incorrect.

This patch avoids the overflow errors by adjusting the value in
write.c:fixup_segment using the VALP pointer in md_apply_fix.

A test for this issue is included.

gas/ChangeLog:

	* config/tc-avr.c (md_apply_fix): Update the contents of VALP for
	diff fixups.

gas/testsuite/ChangeLog:

	* gas/avr/large-debug-line-table.d: New file.
	* gas/avr/large-debug-line-table.s: New file.