While all symbols except for the ones in the version script are hidden,
the exported ones can still be overridden by other libraries which
export the same symbols. Using -fvisibility=protected fixes this. We can
only apply it to the shared library targets because of ld linker issues.

Bug b/110884149

Change-Id: I98222420eabbb9ea0873816f52e1354625b9a3a8
Reviewed-on: https://swiftshader-review.googlesource.com/19688Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>