[libc] Stub handling for the PPC double double type

Summary:
We use the storage class for `long double` in the printing
implementations. We don't fully support the PPC double double type,
which that maps to, but we can stub out just the support needed for the
print interface to works. This required using the internal interface for
storage type, but it should be good enough.

Fixes: #136596

Author: jhuber6

libc/src/__support/FPUtil/FPBits.h

@@ -38,6 +38,7 @@ enum class FPType {
   IEEE754_Binary64,
   IEEE754_Binary128,
   X86_Binary80,
+  PPC_DoubleDouble,
 };
 
 // The classes hierarchy is as follows:
@@ -138,6 +139,15 @@ template <> struct FPLayout<FPType::X86_Binary80> {
   LIBC_INLINE_VAR static constexpr int FRACTION_LEN = SIG_LEN - 1;
 };
 
+// TODO: This needs an FPRepSem interface as well.
+template <> struct FPLayout<FPType::PPC_DoubleDouble> {
+  using StorageType = UInt128;
+  LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
+  LIBC_INLINE_VAR static constexpr int EXP_LEN = 11;
+  LIBC_INLINE_VAR static constexpr int SIG_LEN = 106;
+  LIBC_INLINE_VAR static constexpr int FRACTION_LEN = SIG_LEN - 1;
+};
+
 // FPStorage derives useful constants from the FPLayout above.
 template <FPType fp_type> struct FPStorage : public FPLayout<fp_type> {
   using UP = FPLayout<fp_type>;
@@ -790,6 +800,8 @@ template <typename T> LIBC_INLINE static constexpr FPType get_fp_type() {
       return FPType::IEEE754_Binary64;
     else if constexpr (__LDBL_MANT_DIG__ == 64)
       return FPType::X86_Binary80;
+    else if constexpr (__LDBL_MANT_DIG__ == 106)
+      return FPType::PPC_DoubleDouble;
     else if constexpr (__LDBL_MANT_DIG__ == 113)
       return FPType::IEEE754_Binary128;
   }
@@ -818,7 +830,8 @@ struct FPBits final : public internal::FPRepImpl<get_fp_type<T>(), FPBits<T>> {
   static_assert(cpp::is_floating_point_v<T>,
                 "FPBits instantiated with invalid type.");
   using UP = internal::FPRepImpl<get_fp_type<T>(), FPBits<T>>;
-  using StorageType = typename UP::StorageType;
+  using StorageType =
+      typename internal::FPLayout<get_fp_type<T>()>::StorageType;
 
   // Constructors.
   LIBC_INLINE constexpr FPBits() = default;