emacs: set tab-width always be 8

In c++ and python mode, we use indent with 4 spaces. But if we use
tab-width with 4, the indent become TAB automatically. So we have set
tab-width always be 8, and this is the TAB original define.

Signed-off-by: Micky Ching <mickyching@163.com>
Merge branch 'master' into full