MFC r211706:

MFC r211706:
On shared object unload, in __cxa_finalize, call and clear all installed
atexit and __cxa_atexit handlers that are either installed by unloaded
dso, or points to the functions provided by the dso.

Use _rtld_addr_phdr to locate segment information from the address of
private variable belonging to the dso, supplied by crtstuff.c. Provide
utility function __elf_phdr_match_addr to do the match of address against
dso executable segment.

Call back into libthr from __cxa_finalize using weak
__pthread_cxa_finalize symbol to remove any atfork handler which
function points into unloaded object.

The rtld needs private __pthread_cxa_finalize symbol to not require
resolution of the weak undefined symbol at initialization time. This
cannot work, since rtld is relocated before sym_zero is set up.

MFC r211894:
Do not call __pthread_cxa_finalize with invalid struct dl_phdr_info.

Requested and tested by:	Peter Jeremy <peter rulingia com>

git-svn-id: svn://svn.freebsd.org/base/stable/8@235198 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f