[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

src/boot/memory.cpp

@@ -67,6 +67,8 @@ init_pointer_fixup(uefi::boot_services *bs, uefi::runtime_services *rs)
 	status_line status(L"Initializing pointer virtualization event");
 
 	uefi::event event;
+	bs->set_mem(&fixup_pointers, sizeof(fixup_pointers), 0);
+	fixup_pointer_index = 0;
 
 	try_or_raise(
 		bs->create_event(