add execvp

lib/emscripten/emtests/test_execvp_windows.c

@@ -0,0 +1,18 @@
+#include <stdio.h>
+#include <unistd.h>
+
+int main() {
+    char command[] = "C:\\Windows\\System32\\cmd.exe";
+    char arg1[] = "echo";
+    char arg2[] = "foo";
+    char* argv[4];
+    argv[0] = command;
+    argv[1] = arg1;
+    argv[2] = arg2;
+    argv[3] = 0;
+    printf("_execvp\n");
+    int result = execvp(command, argv);
+    // should not return, and not print this message
+    printf("error");
+    return 0;
+}