[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/support.cpp

@@ -0,0 +1,27 @@
+#include <stdint.h>
+
+#include "error.h"
+
+extern "C" {
+
+/// Basic memcpy() implementation for clang. Clang requires freestanding code
+/// implement memcpy(), as it may emit references to it. This basic memcpy is
+/// not the most efficient, but will get linked if no other memcpy exists.
+__attribute__ ((__weak__))
+void *memcpy(void *dest, const void *src, size_t n)
+{
+	uint8_t *cdest = reinterpret_cast<uint8_t*>(dest);
+	const uint8_t *csrc = reinterpret_cast<const uint8_t*>(src);
+	for (size_t i = 0; i < n; ++i)
+		cdest[i] = csrc[i];
+	return dest;
+}
+
+int _purecall()
+{
+	::boot::error::raise(uefi::status::unsupported, L"Pure virtual call");
+}
+
+} // extern "C"
+
+void operator delete (void *) {}