[release-branch.go1.12] runtime: scan gp._panic in stack scan

In runtime.gopanic, the _panic object p is stack allocated and
referenced from gp._panic. With stack objects, p on stack is dead
at the point preprintpanics runs. gp._panic points to p, but
stack scan doesn't look at gp. Heap scan of gp does look at
gp._panic, but it stops and ignores the pointer as it points to
the stack. So whatever p points to may be collected and clobbered.
We need to scan gp._panic explicitly during stack scan.

To test it reliably, we introduce a GODEBUG mode "clobberfree",
which clobbers the memory content when the GC frees an object.

Fixes #30150.

Change-Id: I11128298f03a89f817faa221421a9d332b41dced
Reviewed-on: https://go-review.googlesource.com/c/161778
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
(cherry picked from commit af8f4062c24cb36af4dc24fbaffd23aa7f7bde36)
Reviewed-on: https://go-review.googlesource.com/c/162358
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
6 files changed
tree: 617acdb97dae6f809a69807c510d03b53c96df9b
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  2. .github/
  3. .gitignore
  4. AUTHORS
  5. CONTRIBUTING.md
  6. CONTRIBUTORS
  7. LICENSE
  8. PATENTS
  9. README.md
  10. VERSION
  11. api/
  12. doc/
  13. favicon.ico
  14. lib/
  15. misc/
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  17. src/
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