Add value_barriers in constant time select functions

The barriers prevent the compiler from narrowing down the
possible value range of the mask and ~mask in the select
statements, which avoids the recognition of the select
and turning it into a conditional load or branch.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
(Merged from https://github.com/openssl/openssl/pull/9217)