Strip LOCAL_CHECK_ELF_FILES

This commit strips LOCAL_CHECK_ELF_FILES before comparing it with
`false` so that developers can add comments after LOCAL_CHECK_ELF_FILES.
For example:

    LOCAL_CHECK_ELF_FILES := false  # disable checks due to ...

Bug: 119084334
Test: Add some spaces after LOCAL_CHECK_ELF_FILES and run
      CHECK_ELF_FILES=true make check-elf-files
Change-Id: I2bfaff4807aeeea6c7af0b9d7d716742b6764cd0