Referring to _DefaultRuneLocale causes this >4KB structure to be copied to
all executables that use <ctype.h> inlines (except PIE executables).
This only affects the case where thread local storage is available.
Reading the lib/libc/locale code, it seems like _CurrentRuneLocale cannot be
NULL (since the non-TLS __getCurrentRuneLocale() and other places use struct
xlocale_ctype.runes without checking), so the check can be removed entirely.
If this reading is incorrect, it seems best to add a check to the assignment
of _CurrentRuneLocale in lib/libc/locale/setrunelocale.c.