Work around sigaltstack() bug on Mac OS X when unregistering a signal stack
Work around sigaltstack() bug on Mac OS X when unregistering a signal stack

This bug manifests itself when building with
"-mmacosx-version-min=10.5", which is necessary in order to build
sel_ldr for x86-64 but which causes a newer version of libc's
sigaltstack() wrapper to be used.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1051
BUG=http://code.google.com/p/nativeclient/issues/detail?id=1053
TEST=run_hello_world_test when SConstruct is changed to use
  "-mmacosx-version-min=10.5" instead of 10.4

Review URL: http://codereview.chromium.org/3857001

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@3504 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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