cmd/go: allow buildmode c-archive for gccgo on ppc64

cmd/go: allow buildmode c-archive for gccgo on ppc64

    In buildmodeinit, the c-archive buildmode is flagged as invalid
    on linux/ppc64 for gccgo when it should be valid. This happens
    because the check against the gccgo flag is done after the checks
    for valid GOOS/GOARCH pairs instead of before as is done for all
    other buildmode cases in this switch. This corrects the problem and
    allows several of the gccgo gotools testcases to pass on linux/ppc64.

    Updates #29046

    Reviewed-on: https://go-review.googlesource.com/c/152137

From-SVN: r266764