aws-lc-sys add support for custom effective_target mappings

[jalil-salame]

Problem:

At work we use a custom target that is equivalent to x86_64-unknown-linux-musl, we found out that by patching aws-lc-sys to map the target like it does for x86_64-alpine-linux-musl compilation works and tests pass, but trying to build the bindings and libraries with cmake fails to include all .S.obj files in the .a archive.

We would like to have some control over this mechanism without having to patch aws-lc-sys or having you guys add a niche target only used by us internally.

Proposed solution:

Add an environment variable to control the effective target.

Some ways the env var could look (with the custom target being x86_64-alpine-linux-musl and the effective target being x86_64-unknown-linux-musl):

• AWS_LC_SYS_EFFECTIVE_TARGET=x86_64-alpine-linux-musl=x86_64-unknown-linux-musl
• AWS_LC_SYS_EFFECTIVE_TARGET_x86_64_alpine_linux_musl=x86_64-unknown-linux-musl

I would be willing to make a PR implementing this functionality.