logger: use gdb commands to dump stack and variable

This patch uses gdb features to dump stack and variable, and
simplifies logger.c a lot.  With this patch, gdb outputs will be
logged to gdb.txt instead of sheep.txt.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>