ggml-cpu : select the NEON fp32->fp16 conversion (#30505)

The __ARM_NEON branch defines both GGML_CPU_COMPUTE_* macros but selects
only FP16_TO_FP32, so FP32_TO_FP16 falls through to the portable
software conversion while neon_compute_fp32_to_fp16, a single fcvt,
stays defined and referenced by nothing. __POWER9_VECTOR__ and
__riscv_zfhmin select both.

Add an #error pair that refuses a branch defining a COMPUTE macro
without selecting it. That is how this stayed unnoticed: the fallback
silently supplies a correct software implementation, so the only
symptom is a hardware instruction nothing ever reaches. The
FP16_TO_FP32 half carries a !defined(__F16C__) carve-out, because PR
30343 keeps that direction on the lookup table on purpose.

Replace the comment above the fallback: it singled out NEON and POWER9
and pointed at a selection that now happens in the branch itself.

ggml/src/ggml-cpu/simd-mappings.h

@@ -42,6 +42,7 @@ extern "C" {
     #define GGML_CPU_COMPUTE_FP32_TO_FP16(x) neon_compute_fp32_to_fp16(x)
 
     #define GGML_CPU_FP16_TO_FP32(x) GGML_CPU_COMPUTE_FP16_TO_FP32(x)
+    #define GGML_CPU_FP32_TO_FP16(x) GGML_CPU_COMPUTE_FP32_TO_FP16(x)
 
     static inline float neon_compute_fp16_to_fp32(ggml_fp16_t h) {
         __fp16 tmp;
@@ -150,9 +151,15 @@ extern float ggml_table_f32_ue4m3[1 << 8];
 #define GGML_CPU_FP32_TO_FP16(x) GGML_CPU_COMPUTE_FP32_TO_FP16(x)
 #endif
 
-// On ARM NEON, it's quicker to directly convert x -> x instead of calling into ggml_lookup_fp16_to_fp32,
-// so we define GGML_CPU_FP16_TO_FP32 and GGML_CPU_FP32_TO_FP16 elsewhere for NEON.
-// This is also true for POWER9.
+// a branch that defines a GGML_CPU_COMPUTE_* macro must also select it, or the fallback takes over silently
+#if defined(GGML_CPU_COMPUTE_FP16_TO_FP32) && !defined(GGML_CPU_FP16_TO_FP32) && !defined(__F16C__) // see: https://github.com/ggml-org/llama.cpp/issues/30342
+#error "a branch defined GGML_CPU_COMPUTE_FP16_TO_FP32 without selecting it as GGML_CPU_FP16_TO_FP32"
+#endif
+#if defined(GGML_CPU_COMPUTE_FP32_TO_FP16) && !defined(GGML_CPU_FP32_TO_FP16)
+#error "a branch defined GGML_CPU_COMPUTE_FP32_TO_FP16 without selecting it as GGML_CPU_FP32_TO_FP16"
+#endif
+
+// fall back to the lookup table / the portable software conversion for whichever direction no branch selected
 #if !defined(GGML_CPU_FP16_TO_FP32)
 inline static float ggml_lookup_fp16_to_fp32(ggml_fp16_t f) {
     uint16_t s;