gcore: Use PT_GETREGSET for NT_THRMISC and NT_PTLWPINFO.

This avoids the need for dealing with converting lwpinfo for alternate
ABIs in gcore itself.

Reviewed by: markj
MFC after: 2 weeks
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D35667

(cherry picked from commit d95657a1a758e8d993a0eb92259f8c860a309207)