Kernel: Modify UART::print_str() to also take into account the length

Previously in the aarch64 Kernel, this would cause dbgln() to actually
print more characters of the next string in memory, because strings in
the Kernel are not zero terminated by default. Prevent this by using the
passed in length of the string.

Kernel/Arch/aarch64/RPi/UART.h

@@ -22,9 +22,9 @@ public:
     void send(u32 c);
     u32 receive();
 
-    void print_str(char const* s)
+    void print_str(char const* s, size_t length)
     {
-        while (*s)
+        for (size_t i = 0; i < length; ++i)
             send(*s++);
     }
     void print_num(u64 n)