Implement popcnt and lzcnt for windows arm64

Fixes: https://github.com/ngtcp2/ngtcp2/issues/261

lib/ngtcp2_cc.c

@@ -244,6 +244,14 @@ static uint64_t ngtcp2_cbrt(uint64_t n) {
 #if defined(_MSC_VER)
 #  if defined(_M_X64)
   d = (int)__lzcnt64(n);
+#  elif defined(_M_ARM64)
+  {
+    unsigned long index;
+    d = sizeof(uint64_t) * CHAR_BIT;
+    if (_BitScanReverse64(&index, n)) {
+      d = d - 1 - index;
+    }
+  }
 #  else
   if ((n >> 32) != 0) {
     d = __lzcnt((unsigned int)(n >> 32));

lib/ngtcp2_ringbuf.c

@@ -31,6 +31,16 @@
 
 #include "ngtcp2_macro.h"
 
+#if defined(_MSC_VER) && defined(_M_ARM64)
+unsigned int __popcnt(unsigned int x) {
+  unsigned int c = 0;
+  for (; x; ++c) {
+    x &= x - 1;
+  }
+  return c;
+}
+#endif
+
 int ngtcp2_ringbuf_init(ngtcp2_ringbuf *rb, size_t nmemb, size_t size,
                         const ngtcp2_mem *mem) {
 #ifdef WIN32