rustc_target: Remove unused Arch::PowerPC64LE
This variant has been added in https://github.com/rust-lang/rust/pull/147645, but actually unused since target_arch for powerpc64le- targets is "powerpc64". (The difference between powerpc64- and powerpc64le- targets is identified by target_endian.)
Note: This is an internal cleanup and does NOT remove `powerpc64le-*` targets.