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>
diff --git a/configure b/configure
index 02c94e2..07b6be3 100755
--- a/configure
+++ b/configure
@@ -4438,7 +4438,7 @@
   aarch64)
     TARGET_BASE_ARCH=arm
     bflt="yes"
-    gdb_xml_files="aarch64-core.xml"
+    gdb_xml_files="aarch64-core.xml aarch64-fpu.xml"
   ;;
   cris)
   ;;