ggml-cpu: gate __fp16 on __ARM_FP16_FORMAT_IEEE (#26860)
* ggml-cpu: gate __fp16 on __ARM_FP16_FORMAT_IEEE
__ARM_NEON only signals NEON availability. The __fp16 type also needs
the IEEE half format, implied on AArch64 but selected with
-mfp16-format=ieee on 32 bit Arm, where the compiler otherwise rejects
the type.
The guard keeps every toolchain that provides the type on the same code
and sends that one configuration to the generic lookup path.
* ggml-cpu: gate the NEON+FMA block on __ARM_FP16_FORMAT_IEEE
Both halves of the F16 section dereference __fp16, so armv7 with
neon-vfpv4 hits the same unknown type error. Without the IEEE
format the configuration now falls back to the scalar path.
Address review from @JonathanC-ARM
