e_os2.h: Refine OSSL_SSIZE definition under UEFI environment

Under UEFI build environment, we may encounter the OSSL_SSIZE macro
re-definition error in e_os2.h if any module call OpenSSL API directly
by including "openssl/xxxx.h" (caused by the predefined _WIN32/_WIN64
macro, which should have been un-defined under OPENSSL_SYS_UEFI).

Though it's not one recommended usage, this patch could still eliminate
the possible build issue by refining the OSSL_SSIZE definition under
OPENSSL_SYS_UEFI.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3121)