Take fcvt.c from musl instead of our modified copy

The only change in our fcvt.c was to circumvent a conflict with glibc's
headers, but this is irrelevant because we no longer use glibc headers.

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