target/arm: Move gdbstub related code out of helper.c

target/arm: Move gdbstub related code out of helper.c

Currently helper.c includes some code which is part of the arm
target's gdbstub support.  This code has a better home: in gdbstub.c
and gdbstub64.c.  Move it there.

Because aarch64_fpu_gdb_get_reg() and aarch64_fpu_gdb_set_reg() move
into gdbstub64.c, this means that they're now compiled only for
TARGET_AARCH64 rather than always.  That is the only case when they
would ever be used, but it does mean that the ifdef in
arm_cpu_register_gdb_regs_for_features() needs to be adjusted to
match.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210921162901.17508-4-peter.maydell@linaro.org