bpo-38858: new_interpreter() reuses pycore_init_builtins() (GH-17351)

new_interpreter() now calls _PyBuiltin_Init() to create the builtins
module and calls _PyImport_FixupBuiltin(), rather than using
_PyImport_FindBuiltin(tstate, "builtins").

pycore_init_builtins() is now responsible to initialize
intepr->builtins_copy: inline _PyImport_Init() and remove this
function.