crypto/threads_lock_contention: factor out obtaining the stack traces data pointer

crypto/threads_lock_contention: factor out obtaining the stack traces data pointer

It also drops the premature initalisation of it in
ossl_init_rwlock_contention_data(), deferring it to on-demand one
in ossl_rwlock_{rd,wr}lock(), which seems to shave some of the incurred
overhead:

    [Before]
    ~/dev/perftools/source$ ./evp_fetch 100
    Average time per fetch call: 16.944004us
    ~/dev/perftools/source$ ./evp_fetch 200
    Average time per fetch call: 26.325767us

    [After]
    ~/dev/perftools/source$ ./evp_fetch 100
    Average time per fetch call: 13.079795us
    ~/dev/perftools/source$ ./evp_fetch 200
    Average time per fetch call: 23.420235us

Signed-off-by: Eugene Syromiatnikov <esyr@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Saša Nedvědický <sashan@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27983)