[ARM, stack protector] If supported, use armv7 instructions.

This commit enables using movt/movw to load the stack guard address:

movw r0, :lower16:(L_g3$non_lazy_ptr-(LPC0_0+8))
movt r0, :upper16:(L_g3$non_lazy_ptr-(LPC0_0+8))
ldr r0, [pc, r0]

Previously a pc-relative load was emitted:

ldr r0, LCPI0_0
ldr r0, [pc, r0]

rdar://problem/18740489

llvm-svn: 220470