Darwin: Do not emit .macinfo when dsymutil cannot consume it.

Darwin: Do not emit .macinfo when dsymutil cannot consume it.

Some verions of dsymutil do not ignore .macinfo sections, but instead
ignore the entire debug in the file.

To avoid this total loss of debug, when we detect that the debug level
is g3 and the dsymutil version cannot support it, we reduce the level
to g2 and issue a note.

This behaviour can be overidden by -gstrict-dwarf (although the objects
will contain macinfo; dsymutil will not produce a .dSYM with it).

gcc/ChangeLog:

	* config/darwin.cc (darwin_override_options): Reduce the debug
	level to 2 if dsymutil cannot handle .macinfo sections.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>
(cherry picked from commit 3c499f8f6f7d19b21d7047efabbe6396ee1c2cac)