Always set the symbol visibility preset to hidden.

[teo-tsirpanis]

Issue #, if available: Fixes #1142

Description of changes:

This PR updates AwsCFlags.cmake to always pass -fvisibility=hidden to the targets, even when we are building static libraries. Instead of directly specifying the compiler option, we use the VISIBILITY_PRESET CMake property, for increased portability across compilers.

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

[graebm]

Sorry for the silence. I've been chipping away at this

• I noticed this change caused CMake to spit out a warning about cmake-policy CMP0063.
• So I made my own branch which did set that policy, but we'd need to update ALL repos (aws-c-http, aws-c-s3, there are more than a dozen others...) to set that policy as well.
• Which made the team wonder why we didn't just update the minimum CMake version, so we wouldn't need to manually set that policy. Updating CMake is something we've wanted to do for years, so now I'm testing that out in the update-cmake branch