[tflite] add xnnpack delegate to label_image

[freedomtan]

rebase and resubmit #36749 to see if it works.

The XNNPACK Delegate uses XNNPACK, a fairly optimized floating point library, to run some inference operators (see the delegate's README for currently supported ops). With XNNPACK, I was able to get performance numbers similar to what @Maratyszcza described at XNNPACK's readme. I also got good numbers on Pixel 4 and Oppo Reno 3. Numbers on x86 machines are also good.

• Single-threaded, label_image -m MODEL_NAME -x 1 -c 50 -t 1

Model	Pixel 3a, ms	Pixel 4, ms	Oppo Reno 3, ms
MobileNet v1 1.0X	88.02	29.95	36.23
MobileNet v2 1.0X	55.13	19.13	21.69
MobileNet v3 Large	44.84	15.69	17.65
MobileNet v3 Small	14.23	5.58	5.66

• multi-threaded, the number of threads is set to be the number of big cores. label_image -m MODEL_NAME -x 1 -c 50 -t NUMBER_OF_BIG_CORES

Model	Pixel 3a, ms	Pixel 4, ms	Oppo Reno 3, ms
MobileNet v1 1.0X	46.00	10.60	13.23
MobileNet v2 1.0X	28.57	6.89	7.56
MobileNet v3 Large	24.62	6.52	6.88
MobileNet v3 Small	8.24	2.47	2.37