amd64: move signal handling and register structures manipulations into exec_machdep.c

from machdep.c which is too large pile of unrelated things.
Some ptrace functions are moved from machdep.c to ptrace_machdep.c.

Now machdep.c contains code mostly related to the low level initialization
and regular low level operation of the architecture, while signal MD code
and registers handling is placed in exec_machdep.c.

Reviewed by: jhb, markj
Discussed with: jrtc27
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31954