Fix running thumb vs arm instruction set on yocto

On yocto CC/CXX export contains flags like -marm, -mthumb
see https://lists.yoctoproject.org/pipermail/poky/2018-September/011497.html



Fix builds for webengine and check also QMAKE_CC for flags,
otherwise build ends up passing -marm and -mthumb flags
at the same time.

Change-Id: I76feb0eaf3bed8a6c3552db1bbf1a0809f03110a
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>