[AVR] Disable 128-bit integers

This commit disables i128 and u128 types in libcore.

Currently there is a code generation bug causing much i128 code to cause
LLVM assertion errors.

The actual error message is here:

```
Unexpected request for libcall!
UNREACHABLE executed at /home/dylan/projects/avr-rust/rust/src/llvm/lib/Target/AVR/AVRISelLowering.cpp:335!
fish: Process 7219, “/home/dylan/projects/avr-rust/rust/build/x86_64-unknown-linux-gnu/stage2/bin/rustc” “~/projects/avr-rust/rust/build/…” terminated by signal SIGABRT (Abort)
```

See avr-rust/rust-legacy-fork#94 for details.

avr-rust/rust-legacy-fork#94

core/lib.rs

@@ -74,7 +74,10 @@
 #![feature(const_fn)]
 #![feature(custom_attribute)]
 #![feature(fundamental)]
-#![feature(i128_type)]
+// AVR: 128-bit integers are not supported.
+// https://github.com/avr-rust/rust/issues/94
+#![cfg_attr(not(target_arch = "avr"), feature(i128_type))]
+
 #![feature(inclusive_range_syntax)]
 #![feature(intrinsics)]
 #![feature(lang_items)]