HEADS UP: switched to gcc 5.4

[Martin Husemann <martin%duskware.de@localhost>]

After quite extensive testing I have switched sparc64 over to new gcc and
binutils again (and while there also new gdb).

Update builds won't work for this change, please make sure to install a full
new build.

If there should be any falloout, as usual please report here or via
send-pr.

Many thanks to everyone helping with the ongoing toolchain struggle!

Martin