Always check ELF files for `check-elf-files`

With this commit, the build system always checks ELF files when
`check-elf-files` is one of the make goals regardless
`PRODUCT_CHECK_ELF_FILES` is specified or not.

This commit also adds `BUILD_BROKEN_PREBUILT_ELF_FILES` for targets that
haven't been cleared.

If this commit breaks your target (usually breaks the targets running
`make checkbuild`), you may triage the breakage by adding the following
setting to your `BoardConfig.mk`:

    BUILD_BROKEN_PREBUILT_ELF_FILES := true

Bug: 141176116
Test: lunch aosp_crosshatch-userdebug && make check-elf-files
Change-Id: Ibe65c977a4ab3d40ba35892ed177eca62c837c35