kata: move test apps into a C hierarchy in prep for adding Rust apps

Note this requires companion changes to the build glue.

Change-Id: I5876d3c8b50f373d21d42cf30dbb7031654fb709
GitOrigin-RevId: 963f05fb3c018ad2d509ef68ef37bf83d924337e

apps/c/hello/hello.c

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2021, Google LLC
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+// This file is a barebones, minimal-dependency test application.
+// It prints the arguments passed in registers to the console
+// using the seL4_DebugPutChar syscall and is intended as a starting
+// point for low-level tests.
+
+#include <kernel/gen_config.h>
+#include <sel4/arch/syscalls.h>
+#include <stdarg.h>
+#include <stdint.h>
+
+__thread seL4_IPCBuffer *__sel4_ipc_buffer;
+
+char minisel_tls[4096] __attribute__((__aligned__(4096)));
+
+__attribute__((naked)) void _start() {
+  asm volatile(
+      ".option push                  \n"
+      ".option norelax               \n"
+      "la gp, __global_pointer$      \n"
+      "la x4, minisel_tls            \n"
+      "addi sp,sp,-16                \n"
+      "sw a0, 12(sp)                 \n"
+      "sw a1, 8(sp)                  \n"
+      "sw a2, 4(sp)                  \n"
+      "sw a3, 0(sp)                  \n"
+      ".option pop                   \n"
+      "j main                        \n");
+}
+
+// only prints 32-bit "%x" hex values
+void minisel_printf(const char *fmt, ...) {
+#if CONFIG_PRINTING
+  va_list args;
+  va_start(args, fmt);
+  for (; *fmt; fmt++) {
+    if (*fmt == '%') {
+      fmt++;
+      if (*fmt == 'x') {
+        uint32_t arg = va_arg(args, uint32_t);
+        for (int i = 7; i >= 0; i--) {
+          int n = (arg >> (4 * i)) & 0xF;
+          seL4_DebugPutChar(n > 9 ? 'A' + n - 10 : '0' + n);
+        }
+      }
+    } else {
+      seL4_DebugPutChar(*fmt);
+    }
+  }
+  va_end(args);
+#endif
+}
+
+int main(int a0, int a1, int a2, int a3) {
+  minisel_printf("\na0 %x a1 %x a2 %x a3 %x\n", a0, a1, a2, a3);
+
+  minisel_printf("Done, sleeping in WFI loop\n");
+  while (1) {
+    asm("wfi");
+  }
+}