tools/nolibc: add option to disable runtime

tools/nolibc: add option to disable runtime

In principle, it is possible to use nolibc for only some object files in
a program. In that case, the startup code in _start and _start_c is not
going to be used. Add the NOLIBC_NO_RUNTIME compile time option to
disable it entirely and also remove anything that depends on it.

Doing this avoids warnings from modpost for UML as the _start_c code
references the main function from the .init.text section while it is not
inside .init itself.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>