binutils/objdump should do the software equivalent of a #UD (emit "(bad)" or whatever is the binutils convention for unrecognized instructions) if f2/f3 prefixes are present before rdrand/rdseed. Intel SDM says so.  Binutils disassembles them as if F2/F3 were legal legacy prefixes for these instructions. 

I demonstrated this with binutils 2.24. I did not check later versions.


% objdump -v
GNU objdump (GNU Binutils) 2.24
Copyright 2013 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or (at your option) any later version.
This program has absolutely no warranty.



% cat foo.c
int main()
{
    asm volatile(".byte 0xF3, 0x0F, 0xC7, 0xF8");
    asm volatile(".byte 0xF3, 0x0F, 0xC7, 0xF0");
    return 0;
}

% gcc foo.c

% objdump -D a.out | grep -A10 'main>:'
0000000000400496 <main>:
  400496:	55                   	push   rbp
  400497:	48 89 e5             	mov    rbp,rsp
  40049a:	f3 0f c7 f8          	repz rdseed eax
  40049e:	f3 0f c7 f0          	repz rdrand eax
  4004a2:	b8 00 00 00 00       	mov    eax,0x0
  4004a7:	5d                   	pop    rbp
  4004a8:	c3                   	ret    
  4004a9:	0f 1f 80 00 00 00 00 	nop    DWORD PTR [rax+0x0]

00000000004004b0 <__libc_csu_init>: