llamafile: powerpc: add FP16 MMA path for Q4/Q8 matmul (llama/19709)

llamafile: powerpc: add FP16 MMA path for Q4/Q8 matmul (llama/19709)

Avoid xvi8ger4pp signed→unsigned bias correction by dequantizing Q4/Q8
inputs to FP16 and using FP16×FP16→FP32 MMA. This removes
post-processing overhead and improves performance.

Performance Impact:
1.5 ~ 2x improvement in PP_Speed for Q4 and Q8 Models,
measured with llama-bench and llama-batched-bench.
Q8 Model: granite-4.0-h-micro-Q8_0.gguf (from huggingface)
Q4 Model: Meta-Llama3-8b Q4 model (generated with llama-quantize from
f32 model)

llama-bench Q8 Model Results:
 model                          	       size 	     params 	 backend    	 threads 	            test 	Base t/s	Patch t/s
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	             pp8 	         64.48 ± 4.72 	         73.99 ± 0.27
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	            pp16 	         80.11 ± 0.32 	        112.53 ± 0.40
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	            pp32 	         89.10 ± 0.27 	        152.95 ± 0.68
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	            pp64 	         93.65 ± 0.25 	        187.83 ± 0.83
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	           pp128 	         99.93 ± 0.02 	        201.32 ± 0.11
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	           pp256 	        102.32 ± 0.40 	        208.32 ± 0.41
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	           pp512 	        103.42 ± 0.40 	        209.98 ± 0.14
 granitehybrid 3B Q8_0          	   3.16 GiB 	     3.19 B 	 CPU        	      10 	           tg128 	         20.35 ± 0.01 	         19.57 ± 0.01

llama-bench Q4 Model Results:
 model                          	       size 	     params 	 backend    	 threads 	            test 	              Base    t/s 	               Patch   t/s
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	             pp8 	         34.77 ± 0.10 	         41.23 ± 0.08
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	            pp16 	         40.81 ± 0.04 	         64.55 ± 0.15
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	            pp32 	         44.65 ± 0.05 	         90.84 ± 0.22
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	            pp64 	         47.49 ± 0.03 	        114.39 ± 0.11
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	           pp128 	         49.29 ± 0.24 	        120.13 ± 0.19
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	           pp256 	         49.77 ± 0.23 	        121.51 ± 0.11
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	           pp512 	         49.89 ± 0.23 	        117.52 ± 0.10
 llama 8B Q4_0                  	   4.33 GiB 	     8.03 B 	 CPU        	      10 	           tg128 	         13.40 ± 0.01 	         13.37 ± 0.00

Llama perplexity Results:

Model	                    Base Final PPL Estimate	Patch Final PPL Estimate
granite-4.0-h-micro-Q8_0    1.3862 +/- 0.04424	        1.3868 +/- 0.04432
Meta-Llama3-8b Q4	    1.3801 +/- 0.04116	        1.3803 +/- 0.04116

Signed-off-by: Shalini.Salomi.Bodapati <redacted>