I have compiled a simple "Hello World" program on DS-5 ultimate evaluation version and couldn't find a bare metal FVP on which I can run it. When I select "ARM FVP->VE_AEMv8x1->Bare Metal Debug->Debug ARMAEMv8-A" in "Connection" tab of "Debug Configuration" connection fails and I see a message that "Unable to connect to armv8_FVP"
I've not noticed that before, but I get it when connecting to the FVP as well. Doesn't seem to stop semihosting from working, my test image (which had a number of semihosting calls) worked without a hitch. Have you tried building and running one of the example projects?
On the putting the cores into stand-by, there are a couple of ways you could do this. A relatively simple method would be to have a check at the beginning of your initialization code. Something like this:
MRS x0, MPIDR_EL1 ; Read MPDIR, which reports affinity AND x0, x0, #0xFF ; Mask to leave Aff0 field CBZ x0, boot ; If Aff0==0 (core 0), branch to boot code sleep WFI ; If Aff0!=0 (cores 1 to 3), go to sleep B sleep boot