Add test program for dummy rocc

Should move this elsewhere

riscv/dummy-rocc-test.c

@@ -0,0 +1,29 @@
+// The following is a RISC-V program to test the functionality of the
+// dummy RoCC accelerator.
+// Compile with riscv-gcc dummy.c
+// Run with spike --extension=dummy pk a.out
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdint.h>
+
+int main() {
+  uint64_t x = 123, y = 456, z = 0;
+  // load x into accumulator 2 (funct=0)
+  asm volatile ("custom0 x0, %0, 2, 0" : : "r"(x));
+  // read it back into z (funct=1) to verify it
+  asm volatile ("custom0 %0, x0, 2, 1" : "=r"(z));
+  assert(z == x);
+  // accumulate 456 into it (funct=3)
+  asm volatile ("custom0 x0, %0, 2, 3" : : "r"(y));
+  // verify it
+  asm volatile ("custom0 %0, x0, 2, 1" : "=r"(z));
+  assert(z == x+y);
+  // do it all again, but initialize acc2 via memory this time (funct=2)
+  asm volatile ("custom0 x0, %0, 2, 2" : : "r"(&x));
+  asm volatile ("custom0 x0, %0, 2, 3" : : "r"(y));
+  asm volatile ("custom0 %0, x0, 2, 1" : "=r"(z));
+  assert(z == x+y);
+
+  printf("success!\n");
+}