target/mips: only build mips-semi for softmmu

target/mips: only build mips-semi for softmmu

The is_uhi gates all semihosting calls and always returns false for
CONFIG_USER_ONLY builds. There is no reason to build and link
mips-semi for these builds so lets fix that.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>