cmd/internal/obj/wasm: increment PC by 2 at sigpanic

On Wasm, PC is not the instruction counter but the block ID. We
advance the PC only when necessary. In the case of sigpanic (used
in nil check), the panic stack trace expects the PC at the call
of sigpanic, not the next one. However, runtime.Caller subtracts
1 from the PC. To make both PC and PC-1 work (have the same line
number), we advance the PC by 2 at sigpanic.

Fixes #29632.

Change-Id: Ieb4d0bb9dc6a8103855a194e3d289f1db4bfb1e5
Reviewed-on: https://go-review.googlesource.com/c/157157


Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>