linux(4): implement coredump support

Implement dumping core for Linux binaries on amd64, for both
32- and 64-bit executables. Some bits are still missing.

This is based on a prototype by chuck@.

Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D30019

(cherry picked from commit 447636e43c08d697664512a50f00f93f41c0a79f)