[IA64] Add __mca_table to the DISCARD list in gate.lds

Add __mca_table to the DISCARD list for the gate.lds linker script to
avoid broken linker references when linking the final vmlinux file.

Also add comment to include/asm-ia64/asmmacros.h to avoid anyone else
hitting this problem in the future.

Credits to James Bottomley <[e-mail]> for spotting
the DISCARD list in gate.lds.S

Signed-off-by: Jes Sorensen <[e-mail]>
Signed-off-by: Tony Luck <[e-mail]>

arch/ia64/kernel/gate.lds.S

···
 	*(.dynbss)
 	*(.bss .bss.* .gnu.linkonce.b.*)
 	*(__ex_table)
+	*(__mca_table)
   }
 }
 

include/asm-ia64/asmmacro.h

···
 
 /*
  * Helper macros for accessing user memory.
+ *
+ * When adding any new .section/.previous entries here, make sure to
+ * also add it to the DISCARD section in arch/ia64/kernel/gate.lds.S or
+ * unpleasant things will happen.
  */
 
 	.section "__ex_table", "a"		// declare section & section attributes