Copy files from ../librthread in preparation for moving functionality
from libpthread to libc. No changes to the build yet, just making it
easier to review the substantive diffs.

ok beck@ kettenis@

Copy files from ../librthread in preparation for moving functionality
from libpthread to libc. No changes to the build yet, just making it
easier to review the substantive diffs.

ok beck@ kettenis@ tedu@

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