baremetal: create an assert_fail C helper

Create a test for it under interactive/assert_fail.c

Move prompt under interactive

Create baremetal/arch/aarch64/c_from_as.S as an example of calling C
functions from assembly.

baremetal/interactive/assert_fail.c

@@ -0,0 +1,6 @@
+#include <common.h>
+
+void main(void) {
+	assert_fail();
+}
+

baremetal/interactive/prompt.c

@@ -0,0 +1,22 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+void main(void) {
+    char c;
+    char *ptr = NULL;
+    size_t alloc_size = 1;
+    while (1) {
+        printf("enter a character\n");
+        c = getchar();
+        printf("got: %c\n", c);
+        ptr = realloc(ptr, alloc_size);
+        if (ptr == NULL) {
+            puts("out of memory");
+            break;
+        } else {
+            printf("new alloc of %d bytes at address 0x%p\n", alloc_size, ptr);
+            alloc_size <<= 1;
+        }
+    }
+}
+