Even though SEH for ARM is incomplete, make what code already exists at least compile correctly.
The _LIBUNWIND_CURSOR_SIZE wasn't correct.
ARM (and AArch64) have a DISPATCHER_CONTEXT field named TargetPc instead of TargetIp.
For the libunwind.h UNW_* constants, there is no UNW_ARM_PC, only UNW_ARM_IP.
Don't use 'r' as loop variable when 'r' already is a Registers_arm member.