Makefile: drop clang option "-integrated-as"

Appears not to be necessary anymore with clang 3.5.
Makefile: drop duplicate "-march=armv6" option for TARGET=PI
Makefile: add variable TARGET_IS_ARMHF

Controls the compiler option "-mfloat-abi=hard".
Makefile: move TARGET_IS_LINUX up

Make it available earlier.
Makefile: use "arm-linux-gnueabi(hf)" for clang/ARM

Instead of specifying the ARMv6/ARMv7 architecture in the "clang
-target" option, use the classic "-march=" flag.  Use a "-target" with
just "arm", so clang can find the linker (there is no ARMv6/ARMv7
specific linker).
Screen/Layout: add unsigned FastScale() overloads
AirspaceListRenderer: don't use PixelScalar
AirspaceListRenderer: cache Layout::FastScale(4)
Dialogs/TaskEditPanel: don't use PixelScalar
Dialogs/Message: don't use PixelScalar