[lld-macho] check minimum header length when opening linkable input files

Bifurcate the `readFile()` API into ...
* `readRawFile()` which performs no checks, and
* `readLinkableFile()` which enforces minimum length of 20 bytes, same as ld64

There are no new tests because tweaks to existing tests are sufficient.

Differential Revision: https://reviews.llvm.org/D97610