Use the copy of the command line that loader_build_argptr() sets up in guest
memory as the command line to return from the ARM SYS_GET_CMDLINE semihosting
call. Previously we were using a pointer to memory which had already been
freed before the guest program started.

This fixes https://bugs.launchpad.net/qemu/+bug/673613 .
Signed-off-by: NWolfgang Schildbach <wschi@dolby.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NRiku Voipio <riku.voipio@iki.fi>