[libc] Fix casts for arm32 after Wconversion (#129771)

Followup to #127523

There were some test failures on arm32 after enabling Wconversion. There
were some tests that were failing due to missing casts. Also I changed
BigInt's `safe_get_at` back to being signed since it needed the ability
to be negative.

Author: michaelrj-google

libc/src/__support/big_int.h

@@ -272,15 +272,15 @@ LIBC_INLINE constexpr cpp::array<word, N> shift(cpp::array<word, N> array,
   if (LIBC_UNLIKELY(offset == 0))
     return array;
   const bool is_neg = is_signed && is_negative(array);
-  constexpr auto at = [](size_t index) -> size_t {
+  constexpr auto at = [](size_t index) -> int {
     // reverse iteration when direction == LEFT.
     if constexpr (direction == LEFT)
-      return N - index - 1;
-    return index;
+      return int(N) - int(index) - 1;
+    return int(index);
   };
   const auto safe_get_at = [&](size_t index) -> word {
     // return appropriate value when accessing out of bound elements.
-    const size_t i = at(index);
+    const int i = at(index);
     if (i < 0)
       return 0;
     if (i >= int(N))
@@ -465,8 +465,7 @@ struct BigInt {
   }
 
   // Initialize the first word to |v| and the rest to 0.
-  template <typename T, typename = cpp::enable_if_t<cpp::is_integral_v<T> &&
-                                                    !cpp::is_same_v<T, bool>>>
+  template <typename T, typename = cpp::enable_if_t<cpp::is_integral_v<T>>>
   LIBC_INLINE constexpr BigInt(T v) {
     constexpr size_t T_SIZE = sizeof(T) * CHAR_BIT;
     const bool is_neg = v < 0;

libc/src/string/memory_utils/generic/byte_per_byte.h

@@ -38,7 +38,8 @@ inline_memmove_byte_per_byte(Ptr dst, CPtr src, size_t count) {
       dst[offset] = src[offset];
   } else {
     LIBC_LOOP_NOUNROLL
-    for (ptrdiff_t offset = count - 1; offset >= 0; --offset)
+    for (ptrdiff_t offset = static_cast<ptrdiff_t>(count - 1); offset >= 0;
+         --offset)
       dst[offset] = src[offset];
   }
 }