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

Review URL: https://breakpad.appspot.com/591002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1173 4c0a9323-5329-0410-9bdc-e9ce6186880e