dlfcn: Failures after dlmopen should not terminate process [BZ #15271]

Commit 9e78f6f6 ("Implement
_dl_catch_error, _dl_signal_error in libc.so [BZ #16628]") has the
side effect that distinct namespaces, as created by dlmopen, now have
separate implementations of the rtld exception mechanism.  This means
that the call to _dl_catch_error from libdl in a secondary namespace
does not actually install an exception handler because the
thread-local variable catch_hook in the libc.so copy in the secondary
namespace is distinct from that of the base namepace.  As a result, a
dlsym/dlopen/... failure in a secondary namespace terminates the process
with a dynamic linker error because it looks to the exception handler
mechanism as if no handler has been installed.

Backport notes: GLRO (dl_catch_error) is replaced with
_dl_catch_error_ptr to preserve _rtld_global_ro layout.

(cherry picked from commit b2964eb1d9a6b8ab1250e8a881cf406182da5875)