add -znotext to kernel module link invocation

ARM kernel modules require .text relocations (DT_TEXTREL) in shared
object ouptut, which is not allowed by default by lld. Add the -znotext
option to enable this. For simplicity add it unconditionally: it is
already default and thus either redundant (GNU BFD ld and gold from
ports) or ignored as an unknown option (GNU BFD ld 2.17.50 in the base
system).

Reviewed by: kib
MFC after: 3 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D11250