[boot] Fix call to exit_boot_services

Exiting boot services can't actually be done from inside
`bootloader_uefi_main`, because there are objects in that scope that run
code requiring boot services in their destructors.

Also added `support.cpp` with `memcpy` because clang will emit
references to `memcpy` even in freestanding mode.

Added a `debug_break` function to allow for faking breakpoints when
connecting to the bootloader with GDB.

Tags: debug

modules.yaml

@@ -62,6 +62,7 @@ modules:
             - src/boot/hardware.cpp
             - src/boot/loader.cpp
             - src/boot/memory.cpp
+            - src/boot/support.cpp
 
     nulldrv:
         kind: exe