target/xtensa: support output to chardev console

In semihosting mode QEMU allows guest to read and write host file
descriptors directly, including descriptors 0..2, a.k.a. stdin, stdout
and stderr. Sometimes it's desirable to have semihosting console
controlled by -serial option, e.g. to connect it to network.

Add semihosting console to xtensa-semi.c, open it in the 'sim' machine
in the presence of -serial option and direct stdout and stderr to it
when it's present.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>