Will upcoming Skylake based MacBooks support 5k displays?
A lot depends on the manufacturer of monitors. Clearly, Intel is backing this as the display technology going forward and Apple is also clearly on board.

Using 4K displays and Ultra HD TVs with your Mac - https://support.apple.com/en-us/HT202856

I think it's early to talk about what a 5k display means - so forgive me if you have a specific resolution and monitor in mind. Apple pulled punches to make their 5K iMac work since the GPU/controller/cables are all internal and they don't have to interoperate. The industry will have to standardize a bit for us to all be comfortable saying 5K display in general.
Currently, you need two cables for the Dell 5K display:

Some displays with resolutions higher than 4K require two DisplayPort cables to connect the display at full resolution. With OS X Yosemite v10.10.3 or later, the Dell UP2715K 27-inch 5K display is supported on the following Mac computers:

Mac Pro (Late 2013)
iMac (Retina 5K, 27-inch, Late 2014 and later)
MacBook Pro (Retina, 15-inch, Mid 2015) with AMD Radeon R9 M370X


That being said, the Late 2013 Mac Pro can drive three 4K displays and doesn't have any of the Thunderbolt 3 advances, so it's inconceivable to me that you won't get two displays over TB3 at 5k and 60 Hz which is currently the gold standard for video workflows that demand high-k displays.
Since nothing is released from Apple saying they Support 5K, you're going to have to read the tea leaves and wait for shipping displays and shipping chipsets with Thunderbolt 3 to know for sure, but I see no reason to be pessimistic about what you want happening on any Mac that ships this fall assuming Thunderbolt 3 over physical USB-C port is part of that Mac.
Intel states that you get 2 4K displays per TB3 connection - so I would presume one TB3 per 5K display would suffice. The single TB3 is listed as DisplayPort 1.2 but has 8 lanes of DisplayPort 1.2 (HBR2 and MST) so I think it's really in the monitor maker's court to get as much of their 5K from one Thunderbolt 3 channel.