ec/curve448: portability fixups.

SPARC condition in __SIZEOF_INT128__==16 is rather performance thing
than portability. Even though compiler advertises int128 capability,
corresponding operations are inefficient, because they are not
directly backed by instruction set.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5449)