Add pkg-host wrapper script

Currently we need pkg-config host for cross-builds to build
build tools. Unfortunately if build environment exports
PKG_CONFIG_* variables pkg-config will pick them up also for host builds,
which can lead to compile errors. Create pkg-config-host_wrapper
which explicitly unsets PKG_CONFIG_* variables.

This is a temporary workaround till proper solution is implemented
in qtbase.

Task-number: QTBUG-65079
Change-Id: I9aff4a27ba62e096ed4c023cf022a41833260178
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>