The CMake build was already using -fvisibility=protected, which has the
same effect but uses duplicate symbol entries for internal/external use.

Bug: b/148240133
Change-Id: I8069a1b293a6ec0604be32179ed591ffaa645ed7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/41648
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>