LuaJIT warnings when building with them enabled

[Buristan]

There are a lot of warnings like:

...
lj_str.c: In function 'lj_str_new':
lj_str.c:184:3: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
  184 |   MSize h = lua_hash(str, len);
      |   ^~~~~
lj_asm.c: In function 'asm_setup_regsp':
lj_asm.c:2132:10: warning: this statement may fall through [-Wimplicit-fallthrough=]
 2132 |       if (REGARG_NUMGPR < 3 && as->evenspill < 3)
      |          ^
lj_asm.c:2141:5: note: here
 2141 |     case IR_CNEW:
      |     ^~~~

...

when build LuaJIT with enable additional warnings:

# Uncomment the next line to enable more warnings:
CCWARN+= -Wextra -Wdeclaration-after-statement -Wredundant-decls -Wshadow -Wpointer-arith

Should we drop some of them like -Wdeclaration-after-statement and fix the others, shouldn't we?