[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.h

@@ -58,6 +58,9 @@ public:
 } // namespace error
 } // namespace boot
 
+/// Debugging psuedo-breakpoint.
+void debug_break();
+
 /// Helper macro to raise an error if an operation fails.
 /// \arg s  An expression evaluating to a UEFI status
 /// \arg m  The error message to use on failure