An embedded trampoline is a function that exists to marshal
a receiver of type *S to a receiver of type *T when T is an
embedded field in S.

Embedded trampolines are generated by a special path through
the compiler and are not subject to the general analysis and
annotation done to functions.  Their effects must be provided
explicitly.

R=golang-dev, r, daniel.morsing, minux.ma
CC=golang-dev
https://golang.org/cl/9874043