libc: use getenv() from musl

While chasing other bugs, I noticed we have our own copy of getenv.c
just so we can add a secure_getenv(). Better just add it in a separate
file, and use the unmodified getenv.c from musl.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>