It requires a handful of additional intrinsics for now.

Fiat's freeze function only works on the tight bounds, so fe_isnonzero
gains an extra fe_carry. But all other calls of fe_tobytes are of tight
bounds anyway.

Change-Id: I834858cee7863c7344e456d7a7dbf4f414f04ae5
Reviewed-on: https://boringssl-review.googlesource.com/24545
Reviewed-by: Adam Langley <agl@google.com>