selftests: kvm: fix compilation

Commit  49de12ba ("selftests: drop KSFT_KHDR_INSTALL make target")
dropped from tools/testing/selftests/lib.mk the code related to KSFT_KHDR_INSTALL,
but in doing so it also dropped the definition of the ARCH variable.  The ARCH
variable is used in several subdirectories, but kvm/ is the only one of these
that was using KSFT_KHDR_INSTALL.

As a result, kvm selftests cannot be built anymore:

In file included from include/x86_64/vmx.h:12,
                 from x86_64/vmx_pmu_caps_test.c:18:
include/x86_64/processor.h:15:10: fatal error: asm/msr-index.h: No such file or directory
   15 | #include <asm/msr-index.h>
      |          ^~~~~~~~~~~~~~~~~

In file included from ../../../../tools/include/asm/atomic.h:6,
                 from ../../../../tools/include/linux/atomic.h:5,
                 from rseq_test.c:15:
../../../../tools/include/asm/../../arch/x86/include/asm/atomic.h:11:10: fatal error: asm/cmpxchg.h: No such file or directory
   11 | #include <asm/cmpxchg.h>
      |          ^~~~~~~~~~~~~~~

Fix it by including the definition that was present in lib.mk.

Fixes: 49de12ba ("selftests: drop KSFT_KHDR_INSTALL make target")
Cc: Guillaume Tucker <guillaume.tucker@collabora.com>
Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>