Add ARM TARGET2 relocation.

According to ARM EHABI, we should use TARGET2 as the
relocation type for type info reference, so we have
to add R_ARM_TARGET2 to MCExpr.

This patch is backported from upstream r167633
by Anton Korobeynikov.

Change-Id: I957fd1c43e0aec7c4b361ad29930fe4358ea610e