Put the arguments of kse_switchin in local variables, rather than

Put the arguments of kse_switchin in local variables, rather than
dereferencing uap throughout. On ia64 uap points into the trapframe
and the call to set_mcontext() in this function will change the
trapframe. Consequently, when we dereference uap afterwards we can
best qualify the behaviour as undefined. By putting the arguments
in local variables we also improve code-generation, because the
compiler is not forced to reload after every function call.

Caught by: Christian Kandeler <christian.kandeler@hob.de>