Fix time_t related warning from clang 3.5 in cpio/cpio.c

Fix the following -Werror warning from clang 3.5.0, while building cpio/cpio.c on amd64 (or any arch with 64-bit time_t):
```
libarchive/cpio/cpio.c:1143:6: error: absolute value function 'abs' given an argument of type 'long' but has parameter of type 'int' which may cause truncation of value [-Werror,-Wabsolute-value]
        if (abs(mtime - now) > (365/2)*86400)
            ^
libarchive/cpio/cpio.c:1143:6: note: use function 'labs' instead
        if (abs(mtime - now) > (365/2)*86400)
            ^~~
            labs
1 error generated.
```
This is because time_t is a long on amd64. To avoid the warning, just copy the equivalent test from a few lines before, which is used in the Windows case, and which is type safe.

Obtained from:	https://reviews.freebsd.org/D1198