hack: TIMEOUT (for controlling test timeout) and forwarding of BUILDFLAGS to binary/dynbinary.

BUILDFLAGS is automatically appended with ORIG_BUILDFLAGS (see comments) so you can just specify what you want. Removing parts of ORIG_BUILDFLAGS is an exercise for the reader.

Note that BUILDFLAGS=-race cannot be used with `make cross` reliably, so I don't use it there and forward ORIG_BUILDFLAGS instead.

already found a few races with BUILDFLAGS=-race. :)

/cc @tianon @tiborvass 
I'd prefer if this were consistent with the rest, ie `-test.timeout="$TIMEOUT"`.