sandbox: define __dyn_sym_start, dyn_sym_end

On RISC-V the symbols __dyn_sym_start, dyn_sym_end are referenced in
efi_runtime_relocate().

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/u-boot.lds

@@ -44,6 +44,13 @@ SECTIONS
 	{
 		*(.__efi_runtime_rel_stop)
 	}
+
+	.dynsym :
+	{
+		__dyn_sym_start = .;
+		*(.dynsym)
+		__dyn_sym_end = .;
+	}
 }
 
 INSERT BEFORE .data;