Enable `ring::agreement` for wasm32 targets using the small
implementation of Curve25519.
Use the small Curve25519 implementation of Curve25519 (and
P-256) for all targets except for Aarch64 and x86-64.
Besides being smaller, the small implementations are likely
more resistant to compiler-introduced side-channels, which
is especially important for the wasm32 virtual machine and
other targets that we don't QA as thoroughly as AAarch64
and x86-64.
Bring in the previously-removed definition of `fe_mul_llt`
from BoringSSL as of commit
8d71d244c0debac4079beeb02b5802fde59b94bd.
