Switch link for ABI to the list of ABIs

This should probably be reworked in the future so the ABI chapter owns
this information. But for now, it doesn't really say anything useful.

Author: ehuss

src/behavior-considered-undefined.md

@@ -257,7 +257,7 @@ reading uninitialized memory is permitted are inside `union`s and in "padding"
 [`const`]: items/constant-items.md
 [noalias]: http://llvm.org/docs/LangRef.html#noalias
 [pointer aliasing rules]: http://llvm.org/docs/LangRef.html#pointer-aliasing-rules
-[abi]: abi.md
+[abi]: items/external-blocks.md#abi
 [undef]: http://llvm.org/docs/LangRef.html#undefined-values
 [`target_feature`]: attributes/codegen.md#the-target_feature-attribute
 [`UnsafeCell<U>`]: std::cell::UnsafeCell