sched: Provide stub definitions for WALT

When WALT and CFI are enabled, weak symbols which points to NULL
are causing issues like global structures becoming NULL due to
compiler/linker treatment of these symbols.

Provide proper stub definition for all such weak symbols to avoid
issues until compiler/linker gets fixed.

Change-Id: I3dce047393521681e663a0ad254b07e9769e0265
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>