kbuild: fix asm-offset generation to work with clang

KBuild abuses the asm statement to write to a file and
clang chokes about these invalid asm statements. Hack it
even more by fooling this is actual valid asm code.

This is an adaption of the Linux kernel commit cf0c3e68aa81f992b0
which in turn is based on a patch for the U-Boot:
  http://patchwork.ozlabs.org/patch/375026/



Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201001072234.143703-5-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Tested-by: Andrew Jones <drjones@redhat.com>