linker: Simplify CTF container handling

Use sys/ctf.h to provide various definitions required to parse the CTF
header. No functional change intended.

Approved by: re (gjb)
Reviewed by: Domagoj Stolfa, emaste
Sponsored by: The FreeBSD Foundation

(cherry picked from commit cab9382a2cdf88b3027370cbdf73b095598eb7c1)
(cherry picked from commit 24597a09b946ccaec4c7e43eae45c81b21d247a3)