Since the introduction of PCAddressMatcher, the executable produced
by the build system is passed to elftools.elf.elffile.ELFFile.
However on macOS, native executables are not ELF files, so the
ELFFile class raises a rather unhelpful AssertionError exception.
Given that the rest of the idf_monitor.py doesn't have assumptions
that the "elf_file" argument is an ELF file (rather than just an
executable), check if the file is a real ELF file inside
PCAddressMatcher.
