lua/utils: use index2adr() instead of index arithmetic

We have a code that performs calculations to obtain the address from the
given index, the address is then passed to cdataV() to get cdata value from
the stack. But this doesn't work for pseudo-indexes (e.g. upvalue indexes).
This patch brings index2adr() from luajit/src/lj_api.c, which accepts all
kinds of indexes, so that the calculations are no longer needed. Also the
helper function luaL_tocpointer() is introduced.

Closes #8249

NO_DOC=bugfix