lib/arm64/spinlock: Fix inline assembly for Clang

/home/travis/build/huth/kvm-unit-tests/lib/arm64/spinlock.c:29:12: error:
 value size does not match register size specified by the constraint and
 modifier [-Werror,-Wasm-operand-widths]
                : "=&r" (val), "=&r" (fail)
                         ^
/home/travis/build/huth/kvm-unit-tests/lib/arm64/spinlock.c:27:9: note: use
 constraint modifier "w"
                "       mov     %0, #1\n"
                                ^~
                                %w0

Use the "w" modifier as suggested to fix the issue.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20201001072234.143703-7-thuth@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Tested-by: Andrew Jones <drjones@redhat.com>