process: replace crucible::gettid() with a weak symbol

process: replace crucible::gettid() with a weak symbol

Since we're now using weak symbols for dodgy libc functions, we might
as well do it for gettid() too.

Use the ::gettid() global namespace and let libc override it.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>