Define xnn_float16 to be _Float16 if we detect that we are compiling … #3167

Job	Run time
build-and-test / cmake-linux-armhf	7m 44s
build-and-test / cmake-android (arm64)	7m 31s
build-and-test / cmake-ios-arm64	4m 28s
build-and-test / cmake-android (armv7)	7m 5s
build-and-test / cmake-android (x86)	9m 20s
build-and-test / cmake-linux-aarch64	8m 11s
build-and-test / cmake-linux-riscv64-rvv	12m 25s
build-and-test / cmake-linux-x86_64	10m 1s
build-and-test / cmake-macos-arm64	9m 32s
build-and-test / cmake-windows-arm64	3m 7s
build-and-test / cmake-windows-x64	5m 41s
build-and-test / cmake-windows-x86	6m 19s
build-and-test / bazel-linux-aarch64	8m 0s
build-and-test / bazel-linux-x86_64-clang-18	8m 17s
build-and-test / bazel-linux-x86_64-gcc-9	6m 53s
build-and-test / cmake-ios-x86_64	7m 43s
build-and-test / cmake-linux-riscv64	6m 42s
	2h 8m 59s

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