[ELF] Set ctx.internalFile for PPC64 _savegpr[01]_{14..31} and _restgpr[01]_{14..31}. NFC

Ensure that every InputFile has a non-null file so that we enforce this
in the future.

Author: MaskRay

lld/ELF/Arch/PPC64.cpp

@@ -286,7 +286,7 @@ static void writeSequence(MutableArrayRef<uint32_t> buf, const char *prefix,
   // The full section content has the extent of [begin, end). We drop unused
   // instructions and write [first,end).
   auto *sec = make<InputSection>(
-      nullptr, SHF_ALLOC, SHT_PROGBITS, 4,
+      ctx.internalFile, SHF_ALLOC, SHT_PROGBITS, 4,
       ArrayRef(reinterpret_cast<uint8_t *>(buf.data() + first),
                4 * (buf.size() - first)),
       ".text");