[libc] Add setgroups and initgroups entrypoints (#227272)

Add the setgroups and initgroups entrypoints from <grp.h> (GNU
extensions).

A Linux syscall wrapper is added for setgroups, supporting both
SYS_setgroups and SYS_setgroups32. The initgroups entrypoint initialises
the supplementary group access list for a specified user by reading the
group database using a scoped database stream and setting the resulting
groups via setgroups.

Assisted-by: Automated tooling, human reviewed.
GitOrigin-RevId: 98f3dda3a7364bd022d2cd7d06ef5e26453fa2a8