Fix warning when running scripts/build_ios.sh (#49457)
Summary:
* Fixes `cmake implicitly converting 'string' to 'STRING' type`
* Fixes `clang: warning: argument unused during compilation: '-mfpu=neon-fp16' [-Wunused-command-line-argument]`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49457
Reviewed By: zhangguanheng66
Differential Revision: D25871014
Pulled By: malfet
fbshipit-source-id: fa0c181ae7a1b8668e47f5ac6abd27a1c735ffce
