Make x86-64 stack walking work for Native Client minidumps

Make x86-64 stack walking work for Native Client minidumps

For NaCl, a stack walker should ignore the top 32 bits of %rip, %rsp
and %rbp, otherwise it will try to read from %r15-extended stack
addresses and look up symbol info for %r15-extended code addresses,
which will fail.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3424
TEST=tested manually with a NaCl minidump

Committed: https://code.google.com/p/google-breakpad/source/detail?r=1173

[mseaborn]

I had a brief look at stackwalker_amd64_unittest.cc but wasn't sure how to add a
test for this.

Ideally the stack walker would truncate rip/rsp/rbp when looking at the initial
register state too, but I wasn't sure where that should go.  The NaCl minidumper
will truncate these registers when reporting them, so it's not essential for the
stack walker to handle non-truncated values.

I also haven't tried to change the stack scanner.  Are there cases where we
would have symbol address info without unwind info?

[bradnelson]

LGTM, need mark for owners

[Mark Mentovai]

LGTM

To answer your question, it’s always possible that a stack will run through some
module (maybe a third-party module or a version of a system module we haven’t
seen yet) that we don’t have any dumped symbols for.

[mseaborn]

Committed patchset #1 manually as r1173 (presubmit successful).