Kernel: Demangle userspace ELF symbols in backtraces

Turns out we can use abi::__cxa_demangle() for this, and all we need to
provide is sprintf(), realloc() and free(), so this patch exposes them.

We now have fully demangled C++ backtraces :^)

Kernel/kprintf.cpp

@@ -96,7 +96,7 @@ static void buffer_putch(char*& bufptr, char ch)
     *bufptr++ = ch;
 }
 
-int ksprintf(char* buffer, const char* fmt, ...)
+int sprintf(char* buffer, const char* fmt, ...)
 {
     va_list ap;
     va_start(ap, fmt);