Cleaning UEFI Build with additional OPENSSL_SYS_UEFI flags

Add OPENSSL_SYS_UEFI to remove unused syslog and uid stuffs for
more clean UEFI build.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2961)