kbuild doc: a bundle of fixes on makefiles.txt

It does several fixes:
1. move the displaced ld example to its reasonable place.
2. add new example for command gzip.
3. fix 2 number errors.
4. fix format of chapter 7.x, make it looks the same as other chapters.

Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>