elf: Remove _DL_FIRST_PLATFORM

Remove the definitions of _DL_FIRST_PLATFORM as those were only used
in the _DL_HWCAP_PLATFORM definitions and in _dl_string_platform().
Both were removed.

Note: Removed on every architecture despite of powerpc, where
_dl_string_platform() is still used.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>