Update lang/LuaJIT2 to 2.0.4.

    * Fix stack check in narrowing optimization.
    * Fix Lua/C API typecheck error for special indexes.
    * Fix string to number conversion.
    * Fix lexer error for chunks without tokens.
    * Don't compile IR_RETF after CALLT to ff with-side effects.
    * Fix BC_UCLO/BC_JMP join optimization in Lua parser.
    * Fix corner case in string to number conversion.
    * Gracefully handle lua_error() for a suspended coroutine.
    * Avoid error messages when building with Clang.
    * Fix snapshot #0 handling for traces with a stack check on entry.
    * Fix fused constant loads under high register pressure.
    * Invalidate backpropagation cache after DCE.
    * Fix ABC elimination.
    * Fix debug info for main chunk of stripped bytecode.
    * Fix FOLD rule for string.sub(s, ...) == k.
    * Fix FOLD rule for STRREF of SNEW.
    * Fix frame traversal while searching for error function.
    * Prevent GC estimate miscalculation due to buffer growth.
    * Prevent adding side traces for stack checks.
    * Fix top slot calculation for snapshots with continuations.
    * Fix check for reuse of SCEV results in FORL.
    * Add PS Vita port.
    * Fix compatibility issues with Illumos.
    * Fix DragonFly build (unsupported).
    * OpenBSD/x86: Better executable memory allocation for W^X mode.
    * x86: Fix argument checks for ipairs() iterator.
    * x86: lj_math_random_step() clobbers XMM regs on OSX Clang.
    * x86: Fix code generation for unused result of math.random().
    * x64: Allow building with LUAJIT_USE_SYSMALLOC and
      LUAJIT_USE_VALGRIND.
    * x86/x64: Fix argument check for bit shifts.
    * x86/x64: Fix code generation for fused test/arith ops.
    * ARM: Fix write barrier check in BC_USETS.
    * PPC: Fix red zone overflow in machine code generation.
    * PPC: Don't use mcrxr on PPE.
    * Various archs: Fix excess stack growth in interpreter.
    * FFI: Fix FOLD rule for TOBIT + CONV num.u32.
    * FFI: Prevent DSE across ffi.string().
    * FFI: No meta fallback when indexing pointer to incomplete struct.
    * FFI: Fix initialization of unions of subtypes.
    * FFI: Fix cdata vs. non-cdata arithmetic and comparisons.
    * FFI: Fix __index/__newindex metamethod resolution for ctypes.
    * FFI: Fix compilation of reference field access.
    * FFI: Fix frame traversal for backtraces with FFI callbacks.
    * FFI: Fix recording of indexing a struct pointer ctype object
      itself.
    * FFI: Allow non-scalar cdata to be compared for equality by address.
    * FFI: Fix pseudo type conversions for type punning.

(alnsn)