x86 UEFI: Make _NO_FILE_4Uhere_ work w/ BOOTX64.EFI

The `_NO_FILE_4Uhere_` test case is used by the runner scripts to verify
QEMU's configuration. Make it work with EFI/BOOT/BOOTX64.EFI by compling
a minimal EFI binary, called dummy.c that returns immediately.

Signed-off-by: Marc Orr <marcorr@google.com>
Message-Id: <20211116204053.220523-11-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>