Update to boring v5.0.2

The new version of BoringSSL requires a C++ library:
- Android: libc++ is linked statically, like WebRTC does
- Swift: libstdc++ is linked on Linux (and libz is dropped everywhere)
- Everywhere else: the system C++ library will be linked dynamically

And makes SSE2 a requirement for 32-bit Linux, matching 32-bit Android.

LibSignalClient.podspec

@@ -15,7 +15,6 @@ Pod::Spec.new do |s|
 
   s.swift_version    = '5'
   s.platform         = :ios, '15.0'
-  s.libraries        = ['z']
 
   s.source_files = ['swift/Sources/**/*.swift', 'swift/Sources/**/*.m']
   s.preserve_paths = [