[iOS] Fix the fbios armv7 pika build
Summary: UBN task - T80034029
Test Plan:
1. pika-armv7: ` buck build //xplat/caffe2:aten_metalApple --flagfile 'fbsource//fbobjc/mode/ios.py#iphoneos-armv7,pika10,apple_toolchain'`
2. pika-arm64: ` buck build //xplat/caffe2:aten_metalApple --flagfile 'fbsource//fbobjc/mode/ios.py#iphoneos,pika10,apple_toolchain'`
Differential Revision: D25134207
fbshipit-source-id: 68b32dab1fc382ec23d7602e34bb64786cb38254
