Use countl_zero in bigint

include/fmt/format.h

diff --git a/include/fmt/format.h b/include/fmt/format.h
index a2dbdf9..6268192 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -2831,10 +2831,8 @@
   FMT_CONSTEXPR20 void assign_pow10(int exp) {
     FMT_ASSERT(exp >= 0, "");
     if (exp == 0) return *this = 1;
-    // Find the top bit.
-    int bitmask = 1;
-    while (exp >= bitmask) bitmask <<= 1;
-    bitmask >>= 1;
+    int bitmask = 1 << (num_bits<unsigned>() -
+                        countl_zero(static_cast<uint32_t>(exp)) - 1);
     // pow(10, exp) = pow(5, exp) * pow(2, exp). First compute pow(5, exp) by
     // repeated squaring and multiplication.
     *this = 5;