Fix false positive when checking ELF prebuilts in PRODUCT_COPY_FILES

llvm-readobj is valid for other object file types such as PE/COFF, so
some firmware files are wrongly detected as ELF prebuilts. Additionally
check the output of llvm-readobj to avoid such false positives.

Change-Id: I4c4ede747967b4a1213aadaed2f925672d18579c