[libc][NFC] use builder pattern for ErrnoSetterMatcher (#79153)

ErrnoSetterMatcher::returns can be misleading as it does not initialize
the errno. This is made worse as later on there is a switch statement on
the errno comparator using __builtin_unreachable(). This patch make
ErrnoSetterMatcher::returns give back a builder that is nomially
different from ErrnoSetterMatcher.