Fix ARM_CC_FOR_TARGET handling

The previous patch that introduced the arm_cc_for_target procedure
moved the ARM_CC_FOR_TARGET global check to that procedure, but forgot
to tell tcl that ARM_CC_FOR_TARGET is a global.  As a result,
specifying ARM_CC_FOR_TARGET on the command line actually does
nothing.  This fixes it.

Change-Id: I4e33b7633fa665e2f7b8f8c9592a949d74a19153