[PATCH] selftests: kvm: fix compilation

[Paolo Bonzini]

Commit  49de12ba06ef ("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: 49de12ba06ef ("selftests: drop KSFT_KHDR_INSTALL make target")
Cc: Guillaume Tucker <guillaume.tucker@xxxxxxxxxxxxx>
Cc: Anders Roxell <anders.roxell@xxxxxxxxxx>
Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
Cc: linux-kselftest@xxxxxxxxxxxxxxx
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
---
 tools/testing/selftests/kvm/Makefile | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/tools/testing/selftests/kvm/Makefile b/tools/testing/selftests/kvm/Makefile
index 89c9a8c52c5f..4c122f1b1737 100644
--- a/tools/testing/selftests/kvm/Makefile
+++ b/tools/testing/selftests/kvm/Makefile
@@ -4,6 +4,8 @@ include ../../../build/Build.include
 all:
 
 top_srcdir = ../../../..
+include $(top_srcdir)/scripts/subarch.include
+ARCH            ?= $(SUBARCH)
 
 # For cross-builds to work, UNAME_M has to map to ARCH and arch specific
 # directories and targets in this Makefile. "uname -m" doesn't map to
-- 
2.31.1