Rewrite __start and __stop symbol handling

[Hans-Peter Nilsson]

On Fri, 16 Jun 2017, Alan Modra wrote:
> This arranges for __start and __stop symbols to be defined before
> garbage collection, for all target formats.

> ld/

> 	* testsuite/ld-elf/pr21562a.d: Use xfail rather than notarget.
> 	Correct typos and list of xfail targets.

The list of targets looks improved (thanks ;) but the use of
notarget was correct before from the looks of it: the test
wasn't made to work for them as opposed to there being a bug.
Unless I miss something (in which case the comment could be
updated to something more verbose)?

brgds, H-P