Added fuchsia-specific versions of allocateExecutable() and
markExecutable().

Bug: chromium: 778467
Change-Id: I027f1409f7b0343e102f98fcc34f2e93c2d626e4
Reviewed-on: https://swiftshader-review.googlesource.com/c/23408Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Sergey Ulanov <sergeyu@chromium.org>