Add DCT/IDCT gbench #10978

Job	Run time
Warmup caches	28s
Conformance Build AVX3	53s
Conformance Build AVX2	53s
Conformance Build SSE4	57s
Conformance Build SSSE3	59s
Conformance Build EMU128	51s
Conformance Build SCALAR	1m 20s
Conformance Build SCALAR_ASAN	1m 12s
Conformance Test main_level5 on AVX3	43s
Conformance Test main_level5 on AVX2	49s
Conformance Test main_level5 on SSE4	1m 4s
Conformance Test main_level5 on SSSE3	1m 1s
Conformance Test main_level5 on EMU128	53s
Conformance Test main_level5 on SCALAR	48s
Conformance Test main_level5 on SCALAR_ASAN	1m 21s
Conformance Test main_level10 on AVX3	50s
Conformance Test main_level10 on AVX2	48s
Conformance Test main_level10 on SSE4	55s
Conformance Test main_level10 on SSSE3	54s
Conformance Test main_level10 on EMU128	1m 4s
Conformance Test main_level10 on SCALAR	59s
Conformance Test main_level10 on SCALAR_ASAN	1m 18s
	21m 0s

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