vmlinux.lds.h: add linker section for KUnit test suites

Add a linker section where KUnit can put references to its test suites.
This patch is the first step in transitioning to dispatching all KUnit
tests from a centralized executor rather than having each as its own
separate late_initcall.

Co-developed-by: Iurii Zaikin <yzaikin@google.com>
Signed-off-by: Iurii Zaikin <yzaikin@google.com>
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>