Fix breakpoint support in Nios II user-mode emulation.

Fix breakpoint support in Nios II user-mode emulation.

Nios II user-mode emulation was missing handling for EXCP_DEBUG,
making the gdb stub essentially useless.  This patch adds the missing
piece.  The new code was copied from the existing EXCP_TRAP handling
and is also similar to what other targets (e.g., arm) do with EXCP_DEBUG.

Signed-off-by: Sandra Loosemore <sandra@codesourcery.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1550076626-7202-1-git-send-email-sandra@codesourcery.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>