selftests: kvm: remove reassignment of non-absolute variables

Clang's integrated assembler does not allow symbols with non-absolute
values to be reassigned. Modify the interrupt entry loop macro to be
compatible with IAS by using a label and an offset.

Cc: Jian Cai <caij2003@gmail.com>
Signed-off-by: Bill Wendling <morbo@google.com>
References: https://lore.kernel.org/lkml/20200714233024.1789985-1-caij2003@gmail.com/


Message-Id: <20201211012317.3722214-1-morbo@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>