MFC r274846:

Fix the following -Werror warning from clang 3.5.0, while building
usr.bin/cpio on amd64 (or any arch with 64-bit time_t):

contrib/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)

	    ^

contrib/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.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D1198