Fix the binutils test for .NET assembly support so that it distinguishing between...

Fix the binutils test for .NET assembly support so that it distinguishing between targets which do not support the x86 PE format at all, and those that do support it, but which do not recognise the .NET assembly variants.

	* testsuite/binutils-all/objdump.exp
	(test_objdump_dotnet_assemblies): Fix test to distinguish errors
	in parsing simple pei-i386 and pei-x86-64 vs parsing the newly
	introduced machine types.
	* testsuite/gentestdlls.c (write_simple_dll): New function.
	(main): Generate simple and Linux-specific variants of pei-i386
	and pei-x86-64 files so both can be used by tests.