os: fix build tags for dir_regfile.go

os: fix build tags for dir_regfile.go

    We want to build dir_regfile.go if not GNU/linux, and not solaris/386,
    and not solaris/sparc.  The latter two conditions were incorrect.  To
    write ! solaris/386 we have to write !solaris !386.  I forgot De
    Morgan's Law.

    Reviewed-on: https://go-review.googlesource.com/26870

From-SVN: r239393