- put functions and data which are only used before calling the executable's
start function into their own page-aligned segments for unmapping
(only done on amd64, arm64, armv7, powerpc, and sparc64 so far)
- pass .init_array and .preinit_array functions an addition argument which
is a callback to get a structure which includes a function that frees
the boot text and data
- sometimes delay doing RELRO processing: for a shared-object marked
DF_1_INITFIRST do it after the object's .init_array, for the executable
do it after the .preinit_array
- improve test-ld.so to link against libpthread and trigger its initialization
late
libc changes to use this will come later
ok kettenis@

now misalign things.
ok guenther@

This gives fast access to the address on systems that implement
the UserLocal register. TCB caching is still used when running
in the single-threaded mode in order not to penalize old systems.
The kernel counterpart of this change must be in place before
using this diff!
With guenther@

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@

that will be needed and make libpthread work when ld.so/libc.a provide an
initial TIB.