PE image base fallout

PE image base fallout

Fixes x86_64-w64-mingw32 tests that failed with the recent diagnosis
for out of range RVA, and a couple of other gc-sections tests that failed
for other reasons.

	* testsuite/ld-gc/gc.exp: Pass "-image-base 0" to ld for PE, and
	arrange to define __main for some run_dump_test tests.
	* testsuite/ld-gc/pr13683.d: Accept more symbols.
	* testsuite/ld-gc/pr14265.d: Likewise, and ordering.
	* testsuite/ld-scripts/crossref.exp: Pass "-image-base 0" to ld for PE.
	* testsuite/ld-srec/srec.exp: Likewise.
	* testsuite/lib/ld-lib.exp (ld_link_defsyms): Use is_pecoff_format.