Label /system/bin/linker* symlinks as system_file.

Fixes:
avc: denied { read } for comm="init" name="linker_asan64" dev="sda22" ino=1833
scontext=u:r:init:s0 tcontext=u:object_r:system_linker_exec:s0 tclass=lnk_file
permissive=0

Bug: 116486312
Test: adb unroot && adb shell ls -l /system/bin/linker_asan
Change-Id: I754daaf3576e83d516cc9189b8be04dcc41bbc5c