target/mips: Mark switch fallthroughs with interpretable comments

Mark switch fallthroughs with comments, in cases fallthroughs
are intentional.

The comments "/* fall through */" are interpreted by compilers and
other tools, and they will not issue warnings in such cases. For gcc,
the warning is turnend on by -Wimplicit-fallthrough. With this patch,
there will be no such warnings in target/mips directory. If such
warning appears in future, it should be checked if it is intentional,
and, if yes, marked with a comment similar to those from this patch.

The comment must be just before next "case", otherwise gcc won't
understand it.

Backports commit 146dd620db815558938433eb9f57a571d424d2c6 from qemu

qemu/target/mips/translate.c

@@ -14410,8 +14410,8 @@ static void decode_micromips32_opc(CPUMIPSState *env, DisasContext *ctx)
         case SDP:
             check_insn(ctx, ISA_MIPS3);
             check_mips_64(ctx);
-            /* Fallthrough */
 #endif
+            /* fall through */
         case LWP:
         case SWP:
             gen_ldst_pair(ctx, minor, rt, rs, SIMM(ctx->opcode, 0, 12));
@@ -14421,8 +14421,8 @@ static void decode_micromips32_opc(CPUMIPSState *env, DisasContext *ctx)
         case SDM:
             check_insn(ctx, ISA_MIPS3);
             check_mips_64(ctx);
-            /* Fallthrough */
 #endif
+            /* fall through */
         case LWM32:
         case SWM32:
             gen_ldst_multiple(ctx, minor, rt, rs, SIMM(ctx->opcode, 0, 12));
@@ -20259,6 +20259,7 @@ static void decode_opc(CPUMIPSState *env, DisasContext *ctx, bool *insn_need_pat
         case OPC_MTHC1:
             check_cp1_enabled(ctx);
             check_insn(ctx, ISA_MIPS32R2);
+            /* fall through */
         case OPC_MFC1:
         case OPC_CFC1:
         case OPC_MTC1: