linux: Add additional ptracestop only if the debugger is Linux

In 6e66030c4c0, additional ptracestop was added in order
to implement PTRACE_EVENT_EXEC. Make it only apply to cases
where the debugger is a Linux processes; native FreeBSD
debuggers can trace Linux processes too, but they don't
expect that additonal ptracestop.

Fixes: 6e66030c4c0
Reported By: kib
Reviewed By: kib
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D32726

(cherry picked from commit 8bbc0600cc21bbfdc3b8e67199eec4220952b7e3)