elf: Add endianness markup to ld.so.cache (bug 27008)

Use a reserved byte in the new format cache header to indicate whether
the file is in little endian or big endian format.  Eventually, this
information could be used to provide a unified cache for qemu-user
and similiar scenarios.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>