Entry functions need to have default visibility instead of protected
visibility to allow symbol preemption during linking (i.e. when not
using dlsym() to explicitly obtain pointers to entry functions). See:
https://bugs.llvm.org//show_bug.cgi?id=30960

Bug swiftshader:64

Change-Id: Ia2d3ae8fb4faec9a75d811c643e3164bfb7d0423
Reviewed-on: https://swiftshader-review.googlesource.com/10028Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>