libgo: change build procedure to use build tags

libgo: change build procedure to use build tags

    Previously the libgo Makefile explicitly listed the set of files to
    compile for each package.  For packages that use build tags, this
    required a lot of awkward automake conditionals in the Makefile.

    This CL changes the build to look at the build tags in the files.
    The new shell script libgo/match.sh does the matching.  This required
    adjusting a lot of build tags, and removing some files that are never
    used.  I verified that the exact same sets of files are compiled on
    amd64 GNU/Linux.  I also tested the build on i386 Solaris.

    Writing match.sh revealed some bugs in the build tag handling that
    already exists, in a slightly different form, in the gotest shell
    script.  This CL fixes those problems as well.

    The old code used automake conditionals to handle systems that were
    missing strerror_r and wait4.  Rather than deal with those in Go, those
    functions are now implemented in runtime/go-nosys.c when necessary, so
    the Go code can simply assume that they exist.

    The os testsuite looked for dir_unix.go, which was never built for gccgo
    and has now been removed.  I changed the testsuite to look for dir.go
    instead.

    Reviewed-on: https://go-review.googlesource.com/25546

From-SVN: r239189