core: chibios: bootloader: use integer pointers as volatile (qmk#13450)

This prevents gcc from incorrectly trying to validate array bounds.

```
tmk_core/common/chibios/bootloader.c: error: '__builtin_memcpy' offset [0, 21] is out of the bounds [0, 0] [-Werror=array-bounds]
  107 |     __builtin_memcpy((void *) VBAT, (const void *)sys_reset_to_loader_magic, sizeof(sys_reset_to_loader_magic));
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578#c16
Fixes qmk#12925

tmk_core/common/chibios/bootloader.c

@@ -103,7 +103,8 @@ void enter_bootloader_mode_if_requested(void) {
 #        define SCB_AIRCR_VECTKEY_WRITEMAGIC 0x05FA0000
 const uint8_t              sys_reset_to_loader_magic[] = "\xff\x00\x7fRESET TO LOADER\x7f\x00\xff";
 __attribute__((weak)) void bootloader_jump(void) {
-    __builtin_memcpy((void *)VBAT, (const void *)sys_reset_to_loader_magic, sizeof(sys_reset_to_loader_magic));
+    void *volatile vbat = (void *)VBAT;
+    __builtin_memcpy(vbat, (const void *)sys_reset_to_loader_magic, sizeof(sys_reset_to_loader_magic));
     // request reset
     SCB->AIRCR = SCB_AIRCR_VECTKEY_WRITEMAGIC | SCB_AIRCR_SYSRESETREQ_Msk;
 }