Compiler: Remove empty fallback definition for LLVM_BUILTIN_UNREACHABLE

`llvm_unreachable()` and `LLVM_ASSUME_ALIGNED` use
`defined(LLVM_BUILTIN_UNREACHABLE)` to check whether it has a
definition. Remove the fallback added in 26827337 (as a drive-by
when updating the GCC logic) and add a comment to prevent future
mistakes.

Differential Revision: https://reviews.llvm.org/D122167