[software] Add fp16 FFT kernel #999

Job	Run time
tc-gcc	2m 49s
check-opcodes	3s
riscv-isa-sim	13s
tc-llvm	5m 26s
verilator	18s
verilator-model (minpool)	17s
verilator-model (mempool)	19s
build-apps-omp (minpool)	41s
build-apps-omp (mempool)	42s
check-bootrom	21s
build-apps-gcc (minpool)	43s
build-apps-gcc (mempool)	47s
unit-test	3m 7s
run-apps-gcc	0s
tc-halide	1m 15s
build-apps-llvm (minpool)	1m 35s
build-apps-llvm (mempool)	1m 24s
build-apps-halide	0s
run-apps-halide	0s
run-apps-llvm	0s
clean-up-compile-runs (minpool)	3s
clean-up-compile-runs (mempool)	3s
clean-up	2s
	20m 8s

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