Add check-elf-files as a checkbuild prerequisite

This commit adds `check-elf-files` as a `checkbuild` prerequisite so
that prebuilt ELF files can be checked in a `checkbuild` build bot when
`PRODUCT_CHECK_ELF_FILES` is set to `true`.

Bug: 140908693
Test: lunch aosp_walleye-userdebug && make checkbuild
Change-Id: Ifea2f7057eecbf74028e986367f2322ff851f5e4