target/arm/arm-semi: Don't let the guest close stdin/stdout/stderr

target/arm/arm-semi: Don't let the guest close stdin/stdout/stderr

The guest can use the semihosting API to open a handle
corresponding to QEMU's own stdin, stdout, or stderr.
When the guest closes this handle, we should not
close the underlying host stdin/stdout/stderr
the way we would do if the handle corresponded to
a host fd we'd opened on behalf of the guest in SYS_OPEN.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200124172954.28481-1-peter.maydell@linaro.org