[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

scripts/templates/exe.boot.j2

@@ -3,6 +3,7 @@
 {{ super() }}
 
 ccflags = $ccflags $
+    -g3 $
     -DKERNEL_FILENAME=L\"jsix.elf\" $
     -I${srcroot}/external/include $
     -I${srcroot}/external/include/X64