Debug stub: use NaClSignalContext for Win threads
Debug stub: use NaClSignalContext for Win threads

Debug stub: merge common threading code, part 1

- use NaClSignalContext instead of Win CONTEXT;
- copy/paste setting/getting registers from POSIX threads;
- remove code responsible for locating registers in CONTEXT;

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2758
TEST=run_debug_stub_test

Review URL: https://chromiumcodereview.appspot.com/10537002

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@8820 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,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client