Use two libc/time files from musl

Unfortunately, many of the files in libc/time are quite heavily modified
from musl (I am not sure why we did that), but found at least two functions,
strptime() and time(), where our changes from musl were trivial and
unnecessary. So use the musl version for these two files.

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