Move panic into its own translation unit

lib/panic.c

@@ -0,0 +1,8 @@
+#include <uart.h>
+volatile int panicked;
+
+void panic(char *s) {
+    panicked = 1;
+    uart_puts(s);
+    while (1);
+}

lib/panic.h

@@ -0,0 +1,6 @@
+#ifndef KERNEL_PANIC_H
+#define KERNEL_PANIC_H
+
+void panic(char *s);
+
+#endif

lib/spinlock.c

@@ -4,13 +4,11 @@
  */
 
 // #include <lib/stdio.h>
+#include <panic.h>
 #include <proc.h>
 #include <riscv.h>
 #include <spinlock.h>
 
-// void panic(char *s) { for (;;); }
-void panic(char *s) {}
-
 /**
  * The aquire() and release() functions control ownership of the lock.
  * To perform these operations, modern CPU's provide atomic instructions