libc: Do not refer to _DefaultRuneLocale in ctype inlines.

[jilles]

Summary

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.

Test Plan

installworld in a VM and kyua test of bin, usr.bin and lib/libc.

[bdrewery]

Found some other bugs while reading this code too!

[theraven]

This probably means that we no longer need to expose _DefaultRuneLocale in the header.

[jilles]

In D10363#260440, @theraven wrote:

This probably means that we no longer need to expose _DefaultRuneLocale in the header.

libc++ (contrib/libc++/src/locale.cpp) uses _DefaultRuneLocale, so it should remain available for now.

The __isctype inline in include/_ctype.h also refers to _DefaultRuneLocale, but it is not used.