Fix x86/x86_64 getcontext overflow problem.

The x86 getcontext code was writing past the end of the context structure.
Fixed this problem, and added a test to make sure that doesn't happen again.
In addition, the x86_64 getcontext was doing the same.

Bug: 14319200
Change-Id: Ia2e2e55fe5459d6c8f8257e787012546b77503c4
5 files changed