[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/error.cpp

@@ -89,6 +89,8 @@ cpu_assert_handler::handle(uefi::status s, const wchar_t *message)
 } // namespace error
 } // namespace boot
 
-extern "C" int _purecall() { ::boot::error::raise(uefi::status::unsupported, L"Pure virtual call"); }
-void operator delete (void *) {}
-
+void debug_break()
+{
+	volatile int go = 0;
+	while (!go);
+}