Use mbedtlsv2 and architecture independent macOS commands

Fixes #1819 (closed). Header files from mbedtls v2 are used, but brew install mbedtls on macOS installs the latest version, 3.0.0. This updates the README instructions to install mbedtls@2, and the build files to use it. Since mbedtls@2 is keg-only, this MR uses brew --prefix mbedtls@2 so that users can build the project without having to unlink their mbedtls installation, in case they have the latest version.

Additionally, this replaces the architecture-dependent macOS sections for adding Homebrew's paths, since they can be found for either architecture using brew --prefix. This would also allow a user to build with a non-standard Homebrew install.