Don't sanitize linker_set

Don't sanitize linker_set

The assumptions of linker_set don't play nicely with
AddressSanitizer. AddressSanitizer adds a 'redzone' of zeros around
globals (including those in named sections), whereas linker_set
assumes they are all packed consecutively like a pointer array. So:
let's annotate linker_set so that AddressSanitizer ignores it.

Submitted by:	Matthew Bryan <matthew.bryan@isilon.com>
Reviewed by:	kib, rang_acm.org
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D22239