libc/locale: Fix races between localeconv(3) and setlocale(3)

[markj]

Summary

Each locale embeds a lazily initialized lconv which is populated by
localeconv(3) and localeconv_l(3). When setlocale(3) updates the global
locale, the lconv needs to be (lazily) reinitialized. To signal this,
we set flag variables in the locale structure. There are two problems:

- The flags are set before the locale is fully updated, so a concurrent
  localeconv() call can observe partially initialized locale data.
- No barriers ensure that localeconv() observes a fully initialized
  locale if a flag is set.

So, move the flag update appropriately, and use acq/rel barriers to
provide some synchronization.

Thanks to Henry Hu <[e-mail]> for providing a test case
demonstrating the race.

PR: 258360
MFC after: 2 weeks