Support AT_FXRNG and AT_KPRELOAD on FreeBSD.

Support AT_FXRNG and AT_KPRELOAD on FreeBSD.

FreeBSD's kernel has recently added two new ELF auxiliary vector
entries.  AT_FXRNG points to a root seed version for the kernel's
PRNG.  Userland can use this to reseed a userland PRNG after the
kernel's PRNG has reseeded.  AT_KPRELOAD is the base address of a
kernel-provided vDSO.

This change displays the proper name and description of these entries
in 'info auxv'.

include/ChangeLog:

	* elf/common.h (AT_FREEBSD_FXRNG, AT_FREEBSD_KPRELOAD): Define.