vm: add strcmp_ext for use by testcases

Also add a testcase that copies strings to the stack and calls strcmp_ext.

tests/string-stack.data

diff --git a/tests/string-stack.data b/tests/string-stack.data
new file mode 100644
index 0000000..4298dc7
--- /dev/null
+++ b/tests/string-stack.data
@@ -0,0 +1,51 @@
+-- c
+extern int strcmp_ext(const char *a, const char *b);
+
+int entry(int *mem)
+{
+    char a[] = "abcx";
+    char b[] = "abcy";
+
+    if (strcmp_ext(a, a) != 0) {
+        return 1;
+    }
+
+    if (strcmp_ext(a, b) == 0) {
+        return 1;
+    }
+
+    return 0;
+}
+-- asm
+mov r1, 0x78636261
+stxw [r10-8], r1
+mov r6, 0x0
+stxb [r10-4], r6
+stxb [r10-12], r6
+mov r1, 0x79636261
+stxw [r10-16], r1
+mov r1, r10
+add r1, 0xfffffff8
+mov r2, r1
+call 0x4
+mov r1, r0
+mov r0, 0x1
+lsh r1, 0x20
+rsh r1, 0x20
+jne r1, 0x0, +11
+mov r1, r10
+add r1, 0xfffffff8
+mov r2, r10
+add r2, 0xfffffff0
+call 0x4
+mov r1, r0
+lsh r1, 0x20
+rsh r1, 0x20
+mov r0, 0x1
+jeq r1, r6, +1
+mov r0, 0x0
+exit
+-- result
+0x0
+-- no register offset
+call instruction

vm/test.c

diff --git a/vm/test.c b/vm/test.c
index a19a5ec..0fb64b8 100644
--- a/vm/test.c
+++ b/vm/test.c
@@ -228,4 +228,5 @@
     ubpf_register(vm, 1, "memfrob", memfrob);
     ubpf_register(vm, 2, "trash_registers", trash_registers);
     ubpf_register(vm, 3, "sqrti", sqrti);
+    ubpf_register(vm, 4, "strcmp_ext", strcmp);
 }