MFC r258661:
Add sysctl KERN_PROC_SIGTRAMP to retrieve signal trampoline location for the
given process.

Approved by:	re (gjb)
MFC r258660:
Fix sys/sysctl.h use for cc -m32 on amd64.

Approved by:	re (gjb)
MFC r258663:
Use sysctl KERN_PROC_SIGTRAMP to retrieve the signal trampoline
location for the native amd64 ABI.  This fixes unwinding over the
signal frame after trampoline was moved to the shared page.

Approved by:	re (gjb)