That page does not mention Legato 17.6 at all.
Should we assume that it uses the same toolchain as 17.5, which is the same as 16.10.3?
Is there a new version?
Toolchains are not really per Legato version but per product release version.
Assuming you are working with a WP85xx, at this stage there is no Legato 17.xx release for wp85.
So if you use the WP85 Release 14 firmware and toolchain, you should have something that works, but is not validated by the WP85 team.
It also seems that Sierra has updated the download page; Legato 17.06 is now mentioned:
"Release 17.05 & 17.06 Note: The Linux OS Source and Toolchains remain unchanged. Download the previous version (SWI9X15Y_07.12.09.00, 32-bit Toolchain or 64-bit Toolchain) to get started with the 17.05 or 17.06 Release of the Legato Platform. "