Use register %ebp (instead of %esp) when calculating the value of
.raSearchStart in the cases where there are alignment operators in
the program string.

If alignment operators are found in the program string, the current
value of %ebp must be valid and it is the only reliable data point
that can be used for getting to the previous frame.  Previously, the
.raSearchStart calculation was based on %esp and when %esp is aligned
in the current frame (which is a lossy operation) the resulting
.raSearchStart cannot was incorrect.  There is code that is trying to
work around this problem (scanning of up to 3 words for a return
address) which is unreliable and it doesn't work in many cases (e.g.
when the alignment is on a 64-byte boundary).

This fix is already deployed in Google and it was measured to reduce
the number of wrong stack traces (for Windows crashes) by 45%. No
regressions have been found so far.

Here is an example of an issue that was fixed by this change (where
register %esp is aligned on the 64-byte boundary and the workarounds
that we already had didn't work):

https://code.google.com/p/chromium/issues/detail?id=311359

0:013> uf chrome_59630000!base::MessagePumpForIO::DoRunLoop
  518 59685c39 55      push    ebp
  518 59685c3a 8bec    mov     ebp,esp
  518 59685c3c 83e4c0  and     esp,0FFFFFFC0h  <== 64-byte boundary
  518 59685c3f 83ec34  sub     esp,34h
  518 59685c42 53      push    ebx
  518 59685c43 56      push    esi

Program string contains 64-byte alignment:
$T1 .raSearch = $T0 $T1 4 - 64 @ = $ebp $T1 4 - ^ = $eip $T1 ^ = 
$esp $T1 4 + = $20 $T0 56 - ^ =  $23 $T0 60 - ^ =  $24 $T0 64 - ^ =
Review URL: https://breakpad.appspot.com/694002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1232 4c0a9323-5329-0410-9bdc-e9ce6186880e