x86: garbage collect SMM programs

The non-module SMM programs were not being garbage collected
during linking. Do this so that one doesn't have to add dependencies
for unused functions in SMM.

TEST=Interrogated readelf -e smm.elf on both builds as well as diffed
     the symbol table. Runtime testing was not done.

Change-Id: I31991496d92191e540df6340c587eec09c7022b3
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/10219
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
diff --git a/src/cpu/x86/smm/smm.ld b/src/cpu/x86/smm/smm.ld
index 6d33fbe..3fde36e 100644
--- a/src/cpu/x86/smm/smm.ld
+++ b/src/cpu/x86/smm/smm.ld
@@ -48,7 +48,7 @@
 	 */
 	. = 0xa8000 - (( CPUS - 1) * 0x400);
 	.jumptable : {
-		*(.jumptable)
+		KEEP(*(.jumptable));
 	}
 
 	/DISCARD/ : {