The symbol runtime._cgo_panic_internal is defined both as a function
in package runtime and as a (linknamed) variable in package
runtime/cgo. Since we're introducing function ABIs, this is going to
cause problems with resolving the ABI-marked function symbol with the
unmarked data symbol. It's also confusing.

Fix this by declaring runtime._cgo_panic_internal as a function in
runtime/cgo as well and extracting the PC from the function object.

For #27539.

Change-Id: I148a458a600cf9e57791cf4cbe92e79bddbf58d4
Reviewed-on: https://go-review.googlesource.com/c/146821
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>