This stores errno, the cancelation flags, and related bits for each thread
and is allocated by ld.so or libc.a.  This is an ABI break from 5.9-stable!
Make libpthread dlopen'able by moving the cancelation wrappers into libc
and doing locking and fork/errno handling via callbacks that libpthread
registers when it first initializes.  'errno' *must* be declared via
<errno.h> now!
Clean up libpthread's symbol exports like libc.
On powerpc, offset the TIB/TCB/TLS data from the register per the ELF spec.
Testing by various, particularly sthen@ and patrick@
ok kettenis@