{ec/curve25519,poly1305/poly1305}.c: relax pedantic constraint.

As it turns out gcc -pedantic doesn't seem to consider __uint128_t
as non-standard, unlike __int128 that is.

Fix even MSVC warnings in curve25519.c.

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)