linker_set: Disable ASAN only in userspace

KASAN does not insert redzones around global variables and so is not
susceptible to the problem that led to us disabling ASAN for linker set
elements in the first place (see commit fe3d8086fb6f).

Reviewed by: andrew, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30126