vmlinux.lds.h update

Updated after review by Tim Abbott.
- Use HEAD_TEXT_SECTION
- Drop use of section-names.h and delete file
- Introduce EXIT_CALL

Deleting section-names.h required a few simple
updates of init.h

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Tim Abbott <tabbott@ksplice.com>