[Darwin, PPC] Use Darwin9 dylib header for Rosetta builds.

On Darwin10 it's possible to make a 32b PPC build using the
'Rosetta' emulator.  However, these builds need to make use of
Darwin9 crts (for exes and dylibs).  This adds the change to
cater for dylibs.

gcc/ChangeLog:

2019-12-13  Iain Sandoe  <iain@sandoe.co.uk>

	* config/rs6000/darwin.h (DARWIN_DYLIB1_SPEC): New.

From-SVN: r279381