[dev.typeparams] runtime: fix misuse of funcPC

funcPC expects a func value. There are places where we pass an
unsafe.Pointer, which is technically undefined.

In proc.go it is actually representing a func value, so the
expression does the right thing. Cast to a func value so it is
clearer.

In os_freebsd.go it is a raw function pointer. Using funcPC on a
raw function pointer is incorrect. Just use it directly instead.

Change-Id: I3c5d61cea08f0abf5737834b520f9f1b583c1d34
Reviewed-on: https://go-review.googlesource.com/c/go/+/321953


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>