BUG/MINOR: compat: make sure __WORDSIZE is always defined

BUG/MINOR: compat: make sure __WORDSIZE is always defined

-Wundef triggered on a MIPS-based musl build on __WORDSIZE that's used
in ultoa_o() and some Lua initialization. The former will fail to convert
integers larger to 1 billion to proper string in this case. Let's make
sure this macro is defined and fall back to values determined from
__SIZEOF_LONG__ otherwise. A cleaner long-term approach would consist
in removing all remaining occurrences of this macro.

This can be backported to all versions.