async_die_is_recursing: work around GCC v11.x issue on Fedora

async_die_is_recursing: work around GCC v11.x issue on Fedora

This fix corrects an issue found in the `dockerized(pedantic, fedora)` CI
build, first appearing after the introduction of a new version of the Fedora
docker image version. This image includes a version of `glibc` with the
attribute `__attr_access_none` added to `pthread_setspecific` [1], the
implementation of which only exists for GCC 11.X - the version included in
the Fedora image. The attribute requires that the pointer provided in the
second argument of `pthread_getspecific` must, if not NULL, be a pointer to
a valid object. In the usage in `async_die_is_recursing`, `(void *)1` is not
valid, causing the error.

This fix imitates a workaround added in SELinux [2] by using the pointer to
the static `async_die_counter` itself as the second argument to
`pthread_setspecific`. This guaranteed non-NULL, valid pointer matches the
intent of the current usage while not triggering the build error.

[1] https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=a1561c3bbe8
[2] https://lore.kernel.org/all/20211021140519.6593-1-cgzones@googlemail.com/

Co-authored-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Victoria Dye <vdye@github.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>