New port: nettle

This is a complete port of nettle, a low-level cryptographic
library, except for the examples. Examples failed to build on
clang toolchain with an internal compiler error. We are
porting nettle 2.7 rather than 3.0 since gnutls does not
build against 3.0.

TEST=./build_all.sh nettle

Review URL: https://codereview.chromium.org/953493002
3 files changed