internal/bytealg, internal/cpu, internal/poll: portability fixes

internal/bytealg, internal/cpu, internal/poll: portability fixes

    In internal/bytealg correct a +build tag to never build indexbyte_generic.go
    for the gofrontend, where we always use indexbyte_native.go.

    For internal/cpu let the Makefile define CacheLineSize using goarch.sh,
    rather than trying to enumerate all the possibilities in cpu_ARCH.go files.

    In internal/poll call the C fcntl function rather than using SYS_FCNTL.
    Change mksysinfo.sh to ensure that F_GETPIPE_SZ is always defined,
    and check that in internal/poll.

    Reviewed-on: https://go-review.googlesource.com/137256

From-SVN: r264572