Fix reporting of the CloudABI ABI in kdump.

• Advertise the word size for CloudABI ABIs via the SV_LP64 flag. All of the other ABIs include either SV_ILP32 or SV_LP64.
• Fix kdump to not assume a 32-bit ABI if the ABI flags field is non-zero but SV_LP64 isn't set. Instead, only assume a 32-bit ABI if SV_ILP32 is set and fallback to the unknown value of "00" if neither SV_LP64 nor SV_ILP32 is set.

Reviewed by: kib, ed
Differential Revision: https://reviews.freebsd.org/D5560