* FFI: Always fall back to metamethods for cdata length/concat.
* FFI: Add tonumber() specialization for failed conversions.
* build: introduce LUAJIT_ENABLE_CHECKHOOK option
* Fix overflow check in unpack().
* gdb: refactor iteration over frames while dumping stack
* gdb: adjust to support Python 2 (CentOS 7)

Closes #7458
Closes #7655
Needed for #7762
Part of #7230

NO_DOC=LuaJIT submodule bump
NO_TEST=LuaJIT submodule bump