gdb: pull upstream gdb-7.7 branch to master

BUG=None
TEST=None

Change-Id: Iadd292b96ddc6f217f90667ba7717c90c6e3cf67
Reviewed-on: https://chromium-review.googlesource.com/197053
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Yunlian Jiang <yunlian@chromium.org>
Tested-by: Yunlian Jiang <yunlian@chromium.org>