runtime: use of floating point in note handler on plan9/amd64

[0intro]

Since CL 98015, the build is failing on plan9/amd64, because the runtime uses floating point instructions in note handler, which is not allowed on Plan 9.

Building Go cmd/dist using /usr/go-plan9-amd64-bootstrap
Building Go toolchain1 using /usr/go-plan9-amd64-bootstrap.
Building Go bootstrap cmd/go (go_bootstrap) using Go toolchain1.
Building Go toolchain2 using go_bootstrap and Go toolchain1.
go_bootstrap 207030: suicide: sys: floating point in note handler pc=0x201010
go_bootstrap 207033: suicide: sys: floating point in note handler pc=0x201010
go_bootstrap 207032: suicide: sys: floating point in note handler pc=0x201010
go_bootstrap 207031: suicide: sys: floating point in note handler pc=0x201010
go tool dist: FAILED: /tmp/workdir-host-plan9-amd64-0intro/go/pkg/tool/plan9_amd64/go_bootstrap install -gcflags=all= -ldflags=all= -i cmd/asm cmd/cgo cmd/compile cmd/link: exit status: 'go_bootstrap 207030: sys: floating point in note handler pc=0x201010'

See https://build.golang.org/log/35578fe371b89f3daa4e382ed33ad2bfa4d9a175

This issue is happening because in CL 98015, findnull was rewritten so it uses bytes.IndexByte which relies on SSE instructions.

[gopherbot]

Change https://golang.org/cl/100577 mentions this issue: runtime: don't use floating point in findnull on Plan 9