pull gdb 7.6.1 from upstream to master BUG=None TEST=None Change-Id: I17b7f2b056e2cbd841081d1778b9526eddf714bf Reviewed-on: https://chromium-review.googlesource.com/171378 Reviewed-by: Luis Lozano <llozano@chromium.org> Commit-Queue: Yunlian Jiang <yunlian@chromium.org> Tested-by: Yunlian Jiang <yunlian@chromium.org>