Defined out MUTEX attributes not available on NonStop SPT Threads.

Defined out MUTEX attributes not available on NonStop SPT Threads.

Standard Posix Threads (SPT) Threads are an older separate branch of
pthreads that do not support some of the capabilities in the current
Posix User Threads (PUT).

The change also includes a rename of the close field of OSSL_STORE_LOADER
which was causing preprocessor conflicts.

Fixes #15885

Signed-off-by: Randall S. Becker <rsbecker@nexbridge.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15886)