RoutineT::getEntry() now returns the actual function pointer type,
rather than void*.

Also rename "callable" to "function", since a callable is a function-
like object (like RoutineT), while getEntry() returns a C function.

Bug: b/143479561
Change-Id: I879b7f9aa13d05d31390596e5e0c5b23f7f80d7d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/51968
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>