Linux: allow a crashing-only custom SIGSYS handler.
Linux: allow a crashing-only custom SIGSYS handler.

We will forward SIGSYS signals to an existing SIGSYS handler but only if
it is guaranteed to not return.
This constraint may be relaxed in the future.

The goal is to allow the use of a seccomp-bpf sandbox with NaCl.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3580

Review URL: https://chromiumcodereview.appspot.com/19778004

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11933 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client