ARM: VIXL32: Keep jump table base label within range of adr.

Macro assembler instructions between adr and the jump table base label
could cause literal pool or veneer emission, which would push the jump
table start out of adr range. Such a large emission is highly unlikely,
but possible, so prevent this by using an ExactAssemblyScope.

Test: export ART_USE_VIXL_ARM_BACKEND=true && test-art-target
Change-Id: I46a038e452e197d73206b415376b5f2246388317