arm: make _end compiler-generated

This prevents references to _end from generating absolute
relocation records.

This change is binary invariant for ARM targets.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
diff --git a/arch/arm/lib/sections.c b/arch/arm/lib/sections.c
index e35687c..5b30bcb 100644
--- a/arch/arm/lib/sections.c
+++ b/arch/arm/lib/sections.c
@@ -25,3 +25,4 @@
 char __image_copy_end[0] __attribute__((section(".__image_copy_end")));
 char __rel_dyn_start[0] __attribute__((section(".__rel_dyn_start")));
 char __rel_dyn_end[0] __attribute__((section(".__rel_dyn_end")));
+char _end[0] __attribute__((section(".__end")));