hpctoolkit: adjust libunwind dependency (#15099)

Change hpctoolkit's dependency on libunwind from 2018.10.12 to 1.4:.
In libunwind, 2018.10.12 is going away in favor of 1.4-rc1 (they're
nearly identical commits).

Remove the 'gpu' version.  This was a temporary branch that is now
folded into master.