runtime: make pcln table check not trigger next to foreign code

runtime: make pcln table check not trigger next to foreign code

Foreign code can be arbitrarily aligned,
so the function before it can have
arbitrarily much padding.
We can't call pcvalue on values in the padding.

Fixes #11653.

Change-Id: I7d57f813ae5a2409d1520fcc909af3eeef2da131
Reviewed-on: https://go-review.googlesource.com/12550
Reviewed-by: Rob Pike <r@golang.org>