Debug stub: Enable tests on x86-32 Windows

Debug stub: Enable tests on x86-32 Windows

Two recent changes make this possible:

r9466 switches the debug stub to use the Windows debug exception
handler process, which is essential for handling any x86-32
untrusted-code faults on Windows.

r9489 fixes the memory access commands to work on x86-32 when
mem_start != 0 and mem_start < 1GB, which is usually the case on
32-bit Windows.  (In contrast, on Linux, we are using a zero-based
x86-32 sandbox so mem_start == 0.  Before the tests enabled the
zero-based sandbox, the kernel would typically allocate addresses >1GB
for mem_start.)

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2880
TEST=run_debug_stub_test + GDB tests in large_tests

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=9495

[eaeltsin, 2012-08-16 16:43:35]

lgtm if bots are happy

[bradn, 2012-08-16 17:13:59]

lgtm