Update libc_cheri's Makefile a bit to make adding more bits of libc
easier: structure things a bit better, use the same WARNS level as
libc, etc.

Add more string routines from libc that are required for many useful
consumers.

Add commented out SRCS lines that will eventually allow us to hook
up much of stdio -- but not yet.
Rename cheritest method constants for hello world / puts to reflect the
fact that they enter via the cheri_system APIs, rather than the POSIX
APIs.  In the future we'll add more variants that use the POSIX APIs,
and hence won't have a CS_ prefix.