target/mips: Use an exception for semihosting

Within do_interrupt, we hold the iothread lock, which
is required for Chardev access for the console, and for
the round trip for use_gdb_syscalls().

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

target/mips/cpu.h

@@ -1252,8 +1252,9 @@ enum {
     EXCP_MSAFPE,
     EXCP_TLBXI,
     EXCP_TLBRI,
+    EXCP_SEMIHOST,
 
-    EXCP_LAST = EXCP_TLBRI,
+    EXCP_LAST = EXCP_SEMIHOST,
 };
 
 /*