handle dumpbase in offloading, adjust testsuite

handle dumpbase in offloading, adjust testsuite

Pass dumpbase on to mkoffloads and their offload-target compiler runs,
using different suffixes for different offloading targets.
Obey -save-temps in naming temporary files while at that.

Adjust the testsuite offload dump scanning machinery to look for dump
files named under the new conventions, iterating internally over all
configured offload targets, or recognizing libgomp's testsuite's own
iteration.

for  gcc/ChangeLog

	* collect-utils.h (dumppfx): New.
	* collect-utils.c (dumppfx): Likewise.
	* lto-wrapper.c (run_gcc): Set global dumppfx.
	(compile_offload_image): Pass a -dumpbase on to mkoffload.
	* config/nvptx/mkoffload.c (ptx_dumpbase): New.
	(main): Handle incoming -dumpbase.  Set ptx_dumpbase.  Obey
	save_temps.
	(compile_native): Pass -dumpbase et al to compiler.
	* config/gcn/mkoffload.c (gcn_dumpbase): New.
	(main): Handle incoming -dumpbase.  Set gcn_dumpbase.  Obey
	save_temps.  Pass -dumpbase et al to offload target compiler.
	(compile_native): Pass -dumpbase et al to compiler.

for  gcc/testsuite/ChangeLog

	* lib/scanoffload.exp: New.
	* lib/scanoffloadrtl.exp: Load it.  Replace ".o" with ""
	globally, and use scanoffload's scoff wrapper to fill it in.
	* lib/scanoffloadtree.exp: Likewise.

for libgomp/ChangeLog

	* testsuite/lib/libgomp.exp: Load gcc lib scanoffload.exp.
	* testsuite/lib/libgomp-dg.exp: Drop now-obsolete -save-temps.