target-arm/arm-semi.c: Fix broken SYS_WRITE0 via gdb

target-arm/arm-semi.c: Fix broken SYS_WRITE0 via gdb

A spurious trailing "\n" in the gdb syscall format string used
for SYS_WRITE0 meant that gdb would reject the remote syscall,
with the effect that the output from the guest was silently dropped.
Remove the newline so that gdb accepts the packet.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>