Add Nios II semihosting support.

This patch adds support for libgloss semihosting to Nios II bare-metal
emulation.  The specification for the protocol can be found in the
libgloss sources.

Signed-off-by: Sandra Loosemore <sandra@codesourcery.com>
Signed-off-by: Julian Brown <julian@codesourcery.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1554321185-2825-3-git-send-email-sandra@codesourcery.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>