Allow objdump to recognise the System.Runtime.dll files that get shipped with .NET...

Allow objdump to recognise the System.Runtime.dll files that get shipped with .NET Core 2.1.

include	* coff/i386.h (IMAGE_FILE_MACHINE_NATIVE_APPLE_OVERRIDE): Define.
	(IMAGE_FILE_MACHINE_NATIVE_FREEBSD_OVERRIDE): Define.
	(IMAGE_FILE_MACHINE_NATIVE_LINUX_OVERRIDE): Define.
	(IMAGE_FILE_MACHINE_NATIVE_NETBSD_OVERRIDE): Define.
	(I386_APPLE_MAGIC): Define.
	(I386_FREEBSD_MAGIC): Define.
	(I386_LINUX_MAGIC): Define.
	(I386_NETBSD_MAGIC): Define.
	(I386BADMAG): Extend macro to allow new magic numbers.
	* coff/x86_64.h (IMAGE_FILE_MACHINE_NATIVE_APPLE_OVERRIDE): Define.
	(IMAGE_FILE_MACHINE_NATIVE_FREEBSD_OVERRIDE): Define.
	(IMAGE_FILE_MACHINE_NATIVE_LINUX_OVERRIDE): Define.
	(IMAGE_FILE_MACHINE_NATIVE_NETBSD_OVERRIDE): Define.
	(AMD64_APPLE_MAGIC): Define.
	(AMD64_FREEBSD_MAGIC): Define.
	(AMD64_LINUX_MAGIC): Define.
	(AMD64_NETBSD_MAGIC): Define.
	(AMD64BADMAG): Extend macro to allow new magic numbers.

bfd	* coffcode.h (coff_set_arch_mach_hook): Handle I386_APPLE_MAGIC,
	I386_FREEBSD_MAGIC, I386_LINUX_MAGIC, I386_NETBSD_MAGIC,
	AMD64_APPLE_MAGIC, AMD64_FREEBSD_MAGIC, AMD64_LINUX_MAGIC,
	AMD64_NETBSD_MAGIC.
	* peXXigen.c: Add comment about source of .NET magic numbers.

binutils* Makefile.am (AUTOMAKE_OPTIONS): Add subdir-objects
	(GENTESTDLLSPROG): Define.
	(TEST_PROGS): Add GENTESTDLLSPROG.
	* Makefile.in: Regenerate.
	* testsuite/binutils-all/objdump.exp
	(test_objdump_dotnet_assemblies): New proc.
	Run the new proc.
	* testsuite/gentestdlls.c: New source file.