kbuild: add sys_* entries for syscalls in tags

Currently, it is no longer possible to use the tags file to jump to
system call function definitions with sys_foo, because the definitions
are obscured by use of the SYSCALL_DEFINE* macros.

This patch adds the appropriate option to ctags to make it see through
the macro.  Also, it adds the ENTRY() work already done for Exuberant
to Emacs too.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>