Please add QMAKE_HOST to the qmake documentation

There is no mention of the QMAKE_HOST variable in the existing qmake documentation. It has been posted on the Qt Developer Network FAQ so it's a bit embarrassing that this is not actually visible anywhere in either the online documentation or Assistant.

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You can use QMAKE_HOST.arch for this. The QMAKE_HOST variable expresses host information about the machine running qmake and QMAKE_HOST.arch allows you to determine the target architecture. You can use it as follows:

win32-g++:contains(QMAKE_HOST.arch, x86_64):{
do something
}

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