tools/syz-declextract: add heuristic for sigsetsize #14402

Job	Run time
dashboard	3m 6s
build	10m 46s
arch (presubmit_arch_linux)	10m 36s
aux	4m 37s
arch (presubmit_arch_freebsd)	6m 38s
gvisor	4m 3s
old	6m 25s
arch (presubmit_arch_netbsd)	4m 19s
race	6m 20s
arch (presubmit_arch_openbsd)	4m 14s
arch (presubmit_arch_darwin)	4m 27s
race_dashboard	4m 49s
arch (presubmit_arch_windows)	3m 25s
arch (presubmit_arch_executor)	3m 10s
	1h 16m 55s

Provide feedback