Add sys/auxv.h wrapper to include/sys/

The GNU libc testsuite fails to build on powerpc/ppc64/ppc64le with the
following error:

    ../sysdeps/powerpc/test-get_hwcap.c:26:22: fatal error: sys/auxv.h: No such file or director

This is because test-get_hwcap.c includes <sys/auxv.h>, but we don't
provide a wrapper in include/sys. This patch adds one.

Changelog:
	* include/sys/auxv.h: New file.