rseq/selftests: Use weak symbol reference, not definition, to link with glibc

Add "extern" to the glibc-defined weak rseq symbols to convert the rseq
selftest's usage from weak symbol definitions to weak symbol _references_.
Effectively re-defining the glibc symbols wreaks havoc when building with
-fno-common, e.g. generates segfaults when running multi-threaded programs,
as dynamically linked applications end up with multiple versions of the
symbols.

Building with -fcommon, which until recently has the been the default for
GCC and clang, papers over the bug by allowing the linker to resolve the
weak/tentative definition to glibc's "real" definition.

Note, the symbol itself (or rather its address), not the value of the
symbol, is set to 0/NULL for unresolved weak symbol references, as the
symbol doesn't exist and thus can't have a value.  Check for a NULL rseq
size pointer to handle the scenario where the test is statically linked
against a libc that doesn't support rseq in any capacity.

Fixes: 3bcbc209 ("selftests/rseq: Play nice with binaries statically linked against glibc 2.35+")
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/all/87frdoybk4.ffs@tglx