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 or the equivalent
attribute fixes this.

Protected visibility used to not work because of
https://bugs.llvm.org//show_bug.cgi?id=30960, but that doesn't appear
to be an issue any more.

Bug chromium:852537
Bug swiftshader:64

Change-Id: Id0b1197b90baa5f5c68bf2aa107d12f8a9856796
Reviewed-on: https://swiftshader-review.googlesource.com/19728Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>