arm: make __bss_start and __bss_end__ compiler-generated

Turn __bss_start and __bss_end__ from linker-generated
to compiler-generated symbols, causing relocations for
these symbols to change type, from R_ARM_ABS32 to
R_ARM_RELATIVE.

This should have no functional impact, as it affects
references to __bss_start and __bss_end__ only before
relocation, and no such references are done.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
diff --git a/board/actux3/u-boot.lds b/board/actux3/u-boot.lds
index a3bd02b..488e1e7 100644
--- a/board/actux3/u-boot.lds
+++ b/board/actux3/u-boot.lds
@@ -72,13 +72,21 @@
 		*(.dynsym)
 	}
 
-	.bss __rel_dyn_start (OVERLAY) : {
-		__bss_start = .;
+	_end = .;
+
+	.bss_start __rel_dyn_start (OVERLAY) : {
+		KEEP(*(.__bss_start));
+	}
+
+	.bss __bss_start (OVERLAY) : {
 		*(.bss*)
 		 . = ALIGN(4);
-		_end = .;
+		 ___bssend___ = .;
+	}
+	.bss_end ___bssend___ (OVERLAY) : {
+		KEEP(*(.__bss_end__));
 	}
-	__bss_end__ =.;
+
 	/DISCARD/ : { *(.dynstr*) }
 	/DISCARD/ : { *(.dynamic*) }
 	/DISCARD/ : { *(.plt*) }