Add endiannes swapping helper functions

Avoid using compiler functions manually to create a cleaner interface,
and allow swapping the endianness of arbritary lengths.

app/src/main/cpp/skyline/common.h

@@ -311,6 +311,24 @@ namespace skyline {
             return result >> (offset + 4);
         }
 
+        template<size_t N>
+        constexpr std::array<u8, N> SwapEndianness(std::array<u8, N> in) {
+            std::reverse(in.begin(), in.end());
+            return in;
+        }
+
+        constexpr u64 SwapEndianness(u64 in) {
+            return __builtin_bswap64(in);
+        }
+
+        constexpr u32 SwapEndianness(u32 in) {
+            return __builtin_bswap32(in);
+        }
+
+        constexpr u16 SwapEndianness(u16 in) {
+            return __builtin_bswap16(in);
+        }
+
         /**
          * @brief A compile-time hash function as std::hash isn't constexpr
          */

app/src/main/cpp/skyline/crypto/aes_cipher.h

@@ -21,7 +21,7 @@ namespace skyline::crypto {
          */
         static std::array<u8, 0x10> GetTweak(size_t sector) {
             std::array<u8, 0x10> tweak{};
-            size_t le{__builtin_bswap64(sector)};
+            size_t le{util::SwapEndianness(sector)};
             std::memcpy(tweak.data() + 8, &le, 8);
             return tweak;
         }

app/src/main/cpp/skyline/vfs/ctr_encrypted_backing.cpp

@@ -13,7 +13,7 @@ namespace skyline::vfs {
 
     void CtrEncryptedBacking::UpdateCtr(u64 offset) {
         offset >>= 4;
-        size_t le{__builtin_bswap64(offset)};
+        size_t le{util::SwapEndianness(offset)};
         std::memcpy(ctr.data() + 8, &le, 8);
         cipher.SetIV(ctr);
     }

app/src/main/cpp/skyline/vfs/nca.cpp

@@ -81,8 +81,8 @@ namespace skyline::vfs {
                 auto key{!(rightsIdEmpty || useKeyArea) ? GetTitleKey() : GetKeyAreaKey(sectionHeader.encryptionType)};
 
                 std::array<u8, 0x10> ctr{};
-                u32 secureValueLE{__builtin_bswap32(sectionHeader.secureValue)};
+                u32 secureValueLE{util::SwapEndianness(sectionHeader.secureValue)};
-                u32 generationLE{__builtin_bswap32(sectionHeader.generation)};
+                u32 generationLE{util::SwapEndianness(sectionHeader.generation)};
                 std::memcpy(ctr.data(), &secureValueLE, 4);
                 std::memcpy(ctr.data() + 4, &generationLE, 4);