Report added symbols

header-abi-diff sets TranslationUnitDiff::added_elf_functions and
added_elf_objects, but does not report them. This commit defines
CompatibilityStatusIR::ElfExtension for the added ELF symbols.
They are reported as ABI difference if neither -allow-extensions nor
-allow-unreferenced-elf-symbol-changes is specified.

Test: ANDROID_BUILD_TOP=`realpath .` \
      PATH=$PATH:`realpath out/soong/dist/bin` \
      development/vndk/tools/header-checker/tests/test.py
Bug: 323447559
Change-Id: I2e8bfcd660ccfc379e2e37eba961450e13fe9ee9