lib: Bundle debugreg.h from the kernel

x86/vmx_tests.c depends on the kernel header and can't be compiled
otherwise on x86_64-elf gcc on macOS.

Cc: Jim Mattson <jmattson@google.com>
Cc: Peter Shier <pshier@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Cameron Esfahani <dirty@apple.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200901085056.33391-5-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>