Stricter gcstress unwind checks.

Don't retry the unwind unconditionally, it masks race issues.
Retry only if missing proc mmaps are likely cause of the failure.

Fail on most unwind errors.

Stricter instrumentation exit pc check.

Test: test.py -r --gcstress --target --ndebug
Change-Id: Idbed80416279717792160fa4531f196d11e909db
1 file changed