Use MADV_DONTNEED for alternate signal stack.

It seems MADV_FREE still counts toward PSS.

Also add a few checks before the `madvise()` call.

Test: testrunner.py --target --64 --optimizing
Bug: 38383823
Bug: 213757852
Change-Id: I8659f8f0490004eb5caba63d0c913931597393b9
1 file changed