Address some -Wold-style-declaration warnings

gcc's -Wextra pulls in -Wold-style-declaration, which triggers when a
declaration has a storage-class specifier as a non-initial qualifier.
The ISO C formal grammar requires the storage-class to be the first
component of the declaration, if present.

Seeint as the register storage-class specifier does not really have any effect
anymore with modern compilers, remove it entirely while we're here, instead of
fixing up the order.

Interestingly, the gcc devteam warnings do not pull in -Wextra, though
the clang ones do.

[extended tests]

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3239)