Fix typing on call to interlockedExchange for windows

Fix typing on call to interlockedExchange for windows

mingw is complaining on builds about the use of InterlockedExchange on a
uint32_t type, as the input parameter here is expected to be LONG
(defined as signed 32 bit on all versions of windows).

the input value (reader_idx) will never grow larger than the group size
of the lock (nominally 2, but always a reasonably small value), so it
should be safe to just cast it to the appropriate type here.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
(Merged from https://github.com/openssl/openssl/pull/25015)