[libc] Add `IN6_IS_ADDR_{LINK, SITE}LOCAL` (#168207)

This patch introduces two macros in `netinet/in.h`. The redundant tests
for macro values in the testcases have been removed.

NOKEYCHECK=True
GitOrigin-RevId: 62ee2cf0f4e1e2aabd0f348064ffb2725fd2508c
2 files changed