Check ELF files in both paths

This commit moves prebuilt ELF file checks out of the ifeq statement so
that both modules with and without LOCAL_STRIP_MODULE can be checked.

Bug: 123670683
Test: Add `LOCAL_STRIP_MODULE := keep_symbols` to some modules and check
      whether `check_elf_files.timestamp` is generated.
Change-Id: I2f2a7df501910d31621da025ce34101f0d358dc2