Don't pass dummy disk images to gem5

After https://gem5.atlassian.net/browse/GEM5-337:

- gem5 baremetal can run without any disk images, just like QEMU
- gem5 X86 can run with a single disk image and no horrendous dummy vmlinux
  and second disk image

build-gem5

@@ -61,18 +61,7 @@ https://github.com/cirosantilli/linux-kernel-module-cheat-regression#gem5-unit-t
             verbose = ['--verbose', LF]
         else:
             verbose = []
-        if self.env['arch'] == 'x86_64':
-            dummy_img_path = os.path.join(disks_dir, 'linux-bigswap2.img')
-            with open(dummy_img_path, 'wb') as dummy_img_file:
-                zeroes = b'\x00' * (2 ** 16)
-                for i in range(2 ** 10):
-                    dummy_img_file.write(zeroes)
-            self.sh.run_cmd(['mkswap', dummy_img_path, LF])
-            with open(os.path.join(binaries_dir, 'x86_64-vmlinux-2.6.22.9'), 'w'):
-                # This file must always be present, despite --kernel overriding that default and selecting the kernel.
-                # I'm not even joking. No one has ever built x86 gem5 without the magic dist dir present.
-                pass
-        elif self.env['arch'] == 'arm' or self.env['arch'] == 'aarch64':
+        if self.env['is_arm']:
             gem5_system_source_dir = os.path.join(self.env['gem5_source_dir'], 'system')
 
             # dtb