cmd/compile,cmd/link: introduce generic interface call relocations

cmd/compile,cmd/link: introduce generic interface call relocations

To capture the fact that a method was called on a generic interface,
so we can make sure the linker doesn't throw away any implementations
that might be the method called.

See the comment in reflect.go for details.

Fixes #49049

Change-Id: I0be74b6e727c1ecefedae072b149f59d539dc1e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/357835
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dan Scales <danscales@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>