Add fallback for ARM Windows fp16 detection. (#348) Background: On a Windows ARM system, I observed that `cpuinfo_has_arm_fp16_arith()` started to return false after upgrading to a more recent cpuinfo version. In #333, the initialization of `cpuinfo_isa.fp16arith` was updated to use `IsProcessorFeaturePresent(PF_ARM_V82_FP16_INSTRUCTIONS_AVAILABLE)`. I suspect that this is not supported on older Windows versions. This change adds a fallback path to set `cpuinfo_isa.fp16arith` the old way. https://github.com/pytorch/cpuinfo/blob/d3a86a813e2bb49d1eb5841ec12e2b135867ab98/src/arm/windows/init.c#L205-L208

diff --git a/src/arm/windows/init.c b/src/arm/windows/init.c
index 5c0a5f3..a07fbe4 100644
--- a/src/arm/windows/init.c
+++ b/src/arm/windows/init.c

@@ -249,6 +249,14 @@
 	// guarantee that, but it holds in practice.

 	cpuinfo_isa.rdm = dotprod;

 

+	// PF_ARM_V82_FP16_INSTRUCTIONS_AVAILABLE may not be available in older

+	// Windows versions. If fp16arith was not detected with

+	// IsProcessorFeaturePresent(PF_ARM_V82_FP16_INSTRUCTIONS_AVAILABLE), fall

+	// back to using the value of dotprod.

+	if (!cpuinfo_isa.fp16arith) {

+		cpuinfo_isa.fp16arith = dotprod;

+	}

+

 	/* Windows API reports all or nothing for cryptographic instructions. */

 	const bool crypto = IsProcessorFeaturePresent(PF_ARM_V8_CRYPTO_INSTRUCTIONS_AVAILABLE) != 0;

 	cpuinfo_isa.aes = crypto;