[Bug target/68872] -mcpu=powerpc64le does not pass -mpower8 to gas

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68872

--- Comment #4 from Peter Bergner <bergner at gcc dot gnu.org> ---
Fix backported to gcc-5-branch and gcc-4_0-branch too.