ARM: support expanding external symbols in 32-bit moves

This enhances the expansion of the mov32imm pseudo-instruction to support an
external symbol reference.  This is motivated by a simplification of the stack
probe emission for Windows on ARM (and fixing a leak).
ARM: fix memory leak, simplify WoA stack probing

This fixes the memory leak introduced with the initial addition of support for
WoA stack probing.  Now that the pseudo-instruction expansion can handle an
external symbol, use that to generate the load which simplifies the logic as
well as avoids the memory leak.