libgo: fix makefile buglet

libgo: fix makefile buglet

    Fix a small bug in the libgo Makefile recipe that constructs the
    directory from which to pick up libgcc_s.so ; the gccgo invocation
    with -print-libgcc-file-name was missing the flags, which meant that
    for -m32 builds we'd see the 64-bit libgcc dir.

    Reviewed-on: https://go-review.googlesource.com/78836

From-SVN: r254984