target/mips: Move helper_cache() to tcg/sysemu/special_helper.c

Move helper_cache() to tcg/sysemu/special_helper.c.

The CACHE opcode is privileged and is not accessible in user
emulation. However we get a link failure when restricting the
symbol to sysemu. For now, add a stub helper to satisfy linking,
which abort if ever called.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210428170410.479308-25-f4bug@amsat.org>