Allow more generic platform names for linux based platforms.

Embedded platforms are not always called exactly
linux-g++ only. A platform may also be called
linux-arm-gnueabi-g++ or similar.

Fail gracefully if the host architecture is not x86_64.
We require 64bit for linking QtWebEngineCore.

Change-Id: I62c32606517bed6ed4307720d3c95e8019ec134b
Reviewed-by: Andras Becsi <andras.becsi@digia.com>
Reviewed-by: Michael Bruning <michael.bruning@digia.com>

tools/qmake/mkspecs/features/functions.prf

 defineTest(isPlatformSupported) {
   static: return(false)
+  !equals(QMAKE_HOST.arch, "x86_64"): return(false)
   osx:lessThan(QMAKE_XCODE_VERSION, 5.1): return(false)
 
-  linux-g++|win32-msvc2013|macx-clang: return(true)
-  android-g++-b2qt: return(true)
+  linux|win32-msvc2013|macx-clang: return(true)
   return(false)
 }