Fix GCC 6.3.0 warnings

This commit fixes some warnings given by GCC 6.3.0.

* Some `assert(false)` are not recognized as `noreturn`ing. They have
  been replaced with `llvm_unreachable`.
* Added `-Wno-ignored-attributes`: attributes are not part the function
  name mangling, and therefore they might create some problems when they
  are involved in template arguments. We don't care.
* Specializations of `readPointer` functions in `binaryfile.h` are now
  `inline`, so they don't appear as "unused" functions.