Fix C++11 strict string literal const correctness.

Fix C++11 strict string literal const correctness.

String literals are const in C++11, and should only be assigned to
const pointers.

Bug swiftshader:33

Change-Id: Iefe0a1c06d58dffb69d7fbe199a2f0aa8bf3bcb1
Reviewed-on: https://swiftshader-review.googlesource.com/8990
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>