Assume KERN_PROC_PATHNAME is present on FreeBSD hosts.

Assume KERN_PROC_PATHNAME is present on FreeBSD hosts.

FreeBSD kernels have included this sysctl since 6.0 release.  The most
recent release without support is 5.5 which was released in May of
2006.

Both the sysctl and the /proc/<pid>/exe file call the same function in
the kernel to obtain the path (vn_fullpath).  If the sysctl fails, the
readlink call will also fail, so there is no need to keep the readlink
fallback.

gdb/ChangeLog:

	* fbsd-nat.c (fbsd_nat_target::pid_to_exec_file): Always use
	sysctl and remove procfs fallback.