Don't double-define Half functions on aarch64

Lexi Bromfieldrequested to merge
lexinadia/eigen:master into master

What does this implement/fix?

This change fixes a compilation error that occurs when compiling for GPU (CUDA) with an aarch64 (aka Arm64) host. Two sets of Half functions are defined, which conflict with each other. To fix this, the change specifically disables the aarch64 versions during the GPU compile phase.

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