fix .code_signature section error

Author: Tropix126

compiler/rustc_target/src/spec/targets/armv7a_vex_v5_linker_script.ld

@@ -15,11 +15,13 @@ __stack_length = 0x400000;
 It's currently unclear why subtracting anything is necessary, but it fixes memory permission errors.
 0x100 is an arbitrary number that works.
 */
-__heap_end = __user_ram_end - __stack_length - 0x1000;
+__heap_end = __user_ram_end - __stack_length - 0x100;
 
 SECTIONS {
     .code_signature __user_ram_start : {
+        __code_signature_start = .;
         KEEP(*(.code_signature))
+        . = __code_signature_start + 0x20;
     } > USER_RAM = 0
 
     .text __user_ram_start + 0x20 : {