The runtime.typeEquals function is used during typelinksinit to
determine the canonical set of *_type values to use throughout the
runtime. As such, it is run against non-canonical *_type values, that
is, types from modules that are duplicates of a type from another
module that was loaded earlier in the program life.

These non-canonical *_type values sometimes contain pointers. These
pointers are pointing to position-independent data, and so they are set
by ld.so using dynamic relocations when the module is loaded. As such,
the pointer can point to the equivalent memory from a previous module.

This means if typesEqual follows a pointer inside a *_type, it can end
up at a piece of memory from another module. If it reads a typeOff or
nameOff from that memory and attempts to resolve it against the
non-canonical *_type from the later module, it will end up with a
reference to junk memory.

Instead, resolve against the pointer the offset was read from, so the
data is valid.

Fixes #17709.
Should no longer matter after #17724 is resolved in a later Go.

Change-Id: Ie88b151a3407d82ac030a97b5b6a19fc781901cb
Reviewed-on: https://go-review.googlesource.com/32513
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>