Provide a gcc-safe zero pointer

zeroptr is a linker object pointing at 0 that can be used to thwart
GCC's (and other compilers') "dereferencing NULL is undefined"
optimization strategy when it gets in the way.

Change-Id: I6aa6f28283281ebae73d6349811e290bf1b99483
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: https://review.coreboot.org/12294
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>

src/lib/program.ld

@@ -133,6 +133,10 @@ _eprogram = .;
 
 /* Discard the sections we don't need/want */
 
+.zeroptr 0 (NOLOAD) : {
+	zeroptr = .;
+}
+
 /DISCARD/ : {
 	*(.comment)
 	*(.comment.*)