tools: Move x86 syscall number fallbacks to .../uapi/

Move unistd_{32,64}.h from x86/include/asm to x86/include/uapi/asm so
that tools/selftests that install kernel headers, e.g. KVM selftests, can
include non-uapi tools headers, e.g. to get 'struct list_head', without
effectively overriding the installed non-tool uapi headers.

Swapping KVM's search order, e.g. to search the kernel headers before
tool headers, is not a viable option as doing results in linux/type.h and
other core headers getting pulled from the kernel headers, which do not
have the kernel-internal typedefs that are used through tools, including
many files outside of selftests/kvm's control.

Prior to commit cec07f53 ("perf tools: Move syscall number fallbacks
from perf-sys.h to tools/arch/x86/include/asm/"), the handcoded numbers
were actual fallbacks, i.e. overriding unistd_{32,64}.h from the kernel
headers was unintentional.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210901203030.1292304-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>