[libc] Special case PPC double double handling for printf

Summary:
Hard code this to a uint128 so it can be built on PPC.

Author: jhuber6

libc/src/stdio/printf_core/core_structs.h

@@ -55,8 +55,13 @@ struct FormatSection {
   int min_width = 0;
   int precision = -1;
 
-  // Needs to be large enough to hold a long double.
+  // Needs to be large enough to hold a long double. Special case handling for
+  // the PowerPC double double type because it has no FPBits interface.
+#ifdef LIBC_TYPES_LONG_DOUBLE_IS_DOUBLE_DOUBLE
+  Uint128 conv_val_raw;
+#else
   fputil::FPBits<long double>::StorageType conv_val_raw;
+#endif // LIBC_TYPES_LONG_DOUBLE_IS_DOUBLE_DOUBLE
   void *conv_val_ptr;
 
   char conv_name;