bsps/or1k: Keep the exception vector table

The linker script matched the .vector input section without KEEP.  That
section is reached only through the hardware exception vectors, so no
symbol references it and --gc-sections removed it.  The executables ended
up with an empty .vector section and no exception was ever delivered.

This includes the tick timer exception, so the clock tick never fired and
every test which waits for a tick ran until the test runner killed it.
Only the tests which need no tick produced a result at all.

The ticker sample completes in about a second now instead of hitting its
timeout.

Update #5686.

Assisted-by: Claude:claude-opus-5 claude-code
Signed-off-by: Sebastian Huber <sebastian.huber@embedded-brains.de>

bsps/or1k/shared/start/linkcmds.base

@@ -56,7 +56,7 @@ bsp_stack_align = DEFINED (bsp_stack_align) ? bsp_stack_align : 8;
 SECTIONS {
 
   .vector : ALIGN_WITH_INPUT {
-    *(.vector)
+    KEEP (*(.vector))
     . = ALIGN(bsp_vector_table_size);
     bsp_section_vector_end = .;
   } > REGION_VECTOR AT > REGION_VECTOR