cmd/compile: SIGSEGV on openbsd-arm64-jsing builder

[bcmills]

greplogs --dashboard -md -l -e '(?ms)\Aopenbsd-arm64-jsing.*go tool compile: signal: segmentation fault' --since=2021-01-01

2022-01-02T14:27:43-c886143/openbsd-arm64-jsing

--- FAIL: TestImportTypeparamTests (71.90s)
    --- FAIL: TestImportTypeparamTests/issue47713.go (0.21s)
        gcimporter_test.go:52: go tool compile: signal: segmentation fault (core dumped)
        gcimporter_test.go:53: go tool compile issue47713.go failed: exit status 1
FAIL
FAIL	go/internal/gcimporter	80.608s

CC @4a6f656c