tools headers arm64: Fix references to top srcdir in Makefile

Aishwarya reports that KVM selftests for arm64 fail with the following
error:

 | make[4]: Entering directory '/tmp/kci/linux/tools/testing/selftests/kvm'
 | Makefile:270: warning: overriding recipe for target
 | '/tmp/kci/linux/build/kselftest/kvm/get-reg-list'
 | Makefile:265: warning: ignoring old recipe for target
 | '/tmp/kci/linux/build/kselftest/kvm/get-reg-list'
 | make -C ../../../../tools/arch/arm64/tools/
 | make[5]: Entering directory '/tmp/kci/linux/tools/arch/arm64/tools'
 | Makefile:10: ../tools/scripts/Makefile.include: No such file or directory
 | make[5]: *** No rule to make target '../tools/scripts/Makefile.include'.
 |  Stop.

It would appear that this only affects builds from the top-level
Makefile (e.g. make kselftest-all), as $(srctree) is set to ".". Work
around the issue by shadowing the kselftest naming scheme for the source
tree variable.

Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Fixes: 0359c946

 ("tools headers arm64: Update sysreg.h with kernel sources")
Link: https://lore.kernel.org/r/20231027005439.3142015-2-oliver.upton@linux.dev
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>