Use is_pecoff_format in ld testsuite

Use is_pecoff_format in ld testsuite

--image-base 0 is not just for x86_64 mingw.  This patch fixes that,
and a case where a changed LDFLAGS leaked out of one script to the next.

	* testsuite/ld-scripts/align.exp: Use is_pecoff_format.
	* testsuite/ld-scripts/defined.exp: Likewise.
	* testsuite/ld-scripts/provide.exp: Likewise.
	* testsuite/ld-scripts/weak.exp: Likewise.
	* testsuite/ld-scripts/empty-address.exp: Likewise.  Reset LDFLAGS
	on exit.
	* testsuite/ld-scripts/expr.exp: Set LDFLAGS earlier, and with
	--image-base for PE.
	* testsuite/ld-scripts/include.exp: Set LDFLAGS for PE.
	* testsuite/ld-scripts/script.exp: Use is_pecoff_format, and
	set LDFLAGS as well as flags.