configs: Don't include 32-bit-only GDB XML in aarch64 linux configs

configs: Don't include 32-bit-only GDB XML in aarch64 linux configs

The aarch64-linux QEMU usermode binaries can never run 32-bit
code, so they do not need to include the GDB XML for it.
(arm_cpu_register_gdb_regs_for_features() will not use these
XML files if the CPU has ARM_FEATURE_AARCH64, so we will not
advertise to gdb that we have them.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210921162901.17508-2-peter.maydell@linaro.org