Dynamic build for FIPS on non-linux platforms

[justsmth]

Issues:

N/A

Description of changes:

• AWS-LC only supports static FIPS builds on Linux, so previously the FIPS build fails on all other platforms.
• This change now performs a shared/dynamic build for aws-lc-fips-sys when on non-linux platforms.
• Also updated build script logic in aws-lc-sys to potentially support a dynamic build in the future.
• Added FIPS test on Mac to the CI

Call-outs:

• Due to a known issue in cargo, doc-tests fail when AWS-LC is linked as a shared library.
• Several parts of this change were copied from skmcgrail's earlier PR

Testing:

• Tests succeed on MacOS (both x86 and M1) and Linux (x86)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.