target-arm: Support fp registers in gdb stub

target-arm: Support fp registers in gdb stub

Register the aarch64-fpu XML and implement the necessary
read/write handlers so we can support reading and writing
of FP registers in the gdb stub.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>