Add `serde-1.0.219-threads4` benchmark for the parallel frontend #4212

Job	Run time
Test benchmarks (--include cargo-0.87.1,stm32f4-0.15.1, Opt)	7m 7s
Test codegen backend benchmarks	1m 5s
Test on Windows	1m 47s
Test benchmarks (--include cargo-0.87.1,stm32f4-0.15.1, Check,Debug,Doc)	6m 30s
Test profiling	3m 46s
Test benchmarks (--exclude cargo-0.87.1,stm32f4-0.15.1, Opt)	11m 27s
Test benchmarks (--exclude cargo-0.87.1,stm32f4-0.15.1, Check,Debug,Doc)	13m 1s
Database Check	1m 17s
Test runtime benchmarks	4m 40s
Compare page endpoint test	4m 35s
Test and deploy	3m 8s
	58m 23s

Provide feedback