The #ifdef'ry hides that the user-mode implementation of
mips_cpu_do_interrupt() simply sets exception_index = EXCP_NONE.

Add this simple implementation to tcg/user/tlb_helper.c, and
the corresponding Meson machinery to build this file when user
emulation is configured.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-14-f4bug@amsat.org>