fix(trigger): extern label should not be inside switch case block.

[ha0lyu]

extern Decode *prev_s; inside a switch case block, which is not allowed because it is a declaration and not an executable statement.

[ha0lyu]

When I make riscv64-xs-ref_defconfig, error occurred:

+ CC src/isa/riscv64/instr/rvv/vcompute_impl.c
+ CC src/isa/riscv64/instr/rvv/vreg_impl.c
src/isa/riscv64/system/trigger.c: In function ‘trigger_handler’:
src/isa/riscv64/system/trigger.c:328:11: error: a label can only be part of a statement and a declaration is not a statement
  328 |           extern Decode *prev_s;
      |           ^~~~~~
make: *** [/home/xs-env/NEMU/scripts/build.mk:62: /home/xs-env/NEMU/build/obj-riscv64-nemu-interpreter-so/src/isa/riscv64/system/trigger.o] Error 1
make: *** Waiting for unfinished jobs....

cc = gcc version 9.5.0 (Ubuntu 9.5.0-1ubuntu1~22.04)
commit id: fce68dc

[NewPaulWalker]

In our work environment, the GCC version is 11.4.0, and this version allows a declaration to follow a label, which is not allowed in GCC 9.5.0.
Although this PR is okay, I suggest that you update and use GCC 11.4.0.
Additionally, there is another feasible fix for GCC 9.5.0, that is enclose the declaration immediately following the case label with {}.

case TRIG_TYPE_ICOUNT:
{
  extern Decode *prev_s;
  prev_s->pc = cpu.pc;
}

[ha0lyu]

Thanks for your suggestion, bug the code should be robust and not rely solely on a high-version GCC compiler.

case TRIG_TYPE_ICOUNT:
{
  extern Decode *prev_s;
  prev_s->pc = cpu.pc;
}

Yes, this is elegant.