Signal handler: Add a sanity check for correct sigaltstack() usage
Signal handler: Add a sanity check for correct sigaltstack() usage

Add back a sanity check that was removed in r3432 that checks that we
are not running on the untrusted stack.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1545
TEST=run_signal_handler_test platform=x86-64
 * can test by breaking NaClSignalStackRegister() to make it a no-op

Review URL: http://codereview.chromium.org/7361003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@6053 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client