build: fix link error for _BitScanForward64 (#28173)

Problem: The usage of `_BitScanForward64` causes linking to fail on some systems.
Solution: Correctly check if it exists using `check_c_source_compiles`.

src/nvim/math.c

@@ -56,7 +56,7 @@ int xctz(uint64_t x)
   // Use compiler builtin if possible.
 #if defined(__clang__) || (defined(__GNUC__) && (__GNUC__ >= 4))
   return __builtin_ctzll(x);
-#elif defined(_MSC_VER)
+#elif defined(HAVE_BITSCANFORWARD64)
   unsigned long index;
   _BitScanForward64(&index, x);
   return (int)index;