target/mips: Replace gen_exception_end(EXCP_RI) by gen_rsvd_instruction

gen_reserved_instruction() is easier to read than
generate_exception_end(ctx, EXCP_RI), replace it.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20201214183739.500368-12-f4bug@amsat.org>