building aarch64-linux-user with clang and --enable-debug fails with undefined reference to kvm_arm_sve_supported

If you build aarch64-linux-user with clang and --enable-debug you get a compile error:

$ mkdir build/clang-debug
$ (cd build/clang-debug/ && ../../configure --target-list=aarch64-linux-user --enable-debug --cc=clang --disable-docs --disable-tools)
[snip configure output]
$ make -C build/clang-debug -j4
[...]
/usr/bin/ld: libqemu-aarch64-linux-user.a.p/target_arm_cpu64.c.o: in function `cpu_arm_set_sve':
/mnt/nvmedisk/linaro/qemu-from-laptop/qemu/build/clang-debug/../../target/arm/cpu64.c:321:(.text+0x1254): undefined reference to `kvm_arm_sve_supported'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

This happens on both arm and x86 hosts.