[OpenMP][libomptarget] Improve device info printing in NextGen plugins

[OpenMP][libomptarget] Improve device info printing in NextGen plugins

This patch improves the device info printing in the NextGen plugins. The device
info properties are composed of keys, values and units (if necessary). These
properties are pushed into a queue by each vendor-specifc plugin, and later,
these properties are printed processed and printed by the common Plugin
Interface. The printing format is common across the different plugins.

Differential Revision: https://reviews.llvm.org/D148178