Disable irrelevant Cray compiler warnings if cc-silence is enabled

Cray is pretty warning-happy, so disable ones that aren't helpful. Each warning
has a numeric value instead of having named flags to disable specific warnings.
Disable warnings 128 and 1357.

128:  Ignore unreachable code warning. Cray warns about `not_reached()` not
      being reachable in a couple of places because it detects that some loops
      will never terminate.

1357: Ignore warning about redefinition of malloc and friends

With this patch, Cray 8.4.0 and 8.5.1 build cleanly and pass `make check`