Add a flag to make lighthouse portable across machines (#1423)

## Issue Addressed

Closes #1395

## Proposed Changes

* Add a feature to `lighthouse` and `lcli` called `portable` which enables the `portable` feature on our fork of BLST. This feature turns off the `-march=native` C compiler flag that produces binaries highly targeted to the host CPU's instruction set.
* Tweak the `Makefile` so that when the `PORTABLE` environment variable is set to `true`, it compiles with this feature.
* Temporarily enable `PORTABLE=true` in the Docker build so that the image on Docker Hub is portable. Eventually I think we should enable `PORTABLE=true` _only on Docker Hub_, so that users building locally can take advantage of the tasty compiler magic. This seems to be possible by setting a Docker Hub environment variable: https://docs.docker.com/docker-hub/builds/#environment-variables-for-builds

## Additional Info

Tested by compiling on a very new CPU (Intel Core i7-8550U) and copying the binary to a very old CPU (Intel Core i3 530). Before the portability fix, this produced the SIGILL crash described in #1395, and after the fix, it worked smoothly.

I'm in the process of testing the Docker build and running some benches to confirm that the performance penalty isn't too severe.

Dockerfile

@@ -1,8 +1,10 @@
-FROM rust:1.44.1 AS builder
+FROM rust:1.45.1 AS builder
 RUN apt-get update && apt-get install -y cmake
 COPY . lighthouse
+ARG PORTABLE
+ENV PORTABLE $PORTABLE
 RUN cd lighthouse && make
-RUN cd lighthouse && cargo install --path lcli --locked
+RUN cd lighthouse && make install-lcli
 
 FROM debian:buster-slim
 RUN apt-get update && apt-get install -y --no-install-recommends \