C6x: fix return from kernel_thread()

- Do not resched or work signals when returning form kernel_thread()
- On kernel_thread(), force new CSR/TSR to have GIE cleared to avoid accidentally
  enabling interrupts when restoring CSR/TSR.
- Move arch_start_context_switch() from pgtable.h to system.h
- Remove dead definitions in system.h
- Reorg MASK_INT, UNMASK_INT macros in entry.S
- Cleanup of entry.S

Signed-off-by: Aurelien Jacquiot <a-jacquiot@ti.com>