add decoder support for endbr64 and endbr32

These show up in ld.so on my machine:

<spurious rep/repne prefix @0x00007f6e2d383f30 (f3 0f 1e fa): >

new executable vm area: 0x00007f040b6c1000-0x00007f040b8c4110 ----  libdl.so.2
<...>
interp: start_pc = 0x00007f040b6c1f30
check_thread_vm_area: pc = 0x00007f040b6c1f30
prepend_entry_to_fraglist: putting fragment @0x00007f040b6c1f30 (shared) on vmarea 0x00007f040b6c1000-0x00007f040b8c3000
check_thread_vm_area: check_stop = 0x00007f040b8c3000
SYSLOG_WARNING: spurious rep/repne prefix @0x00007f040b6c1f30 (f3 0f 1e fa): 
  0x00007f040b6c1f30  f3 0f 1e fa          nop    %edx
  0x00007f040b6c1f34  e9 77 ff ff ff       jmp    $0x00007f040b6c1eb0
interp: direct jump at 0x00007f040b6c1f34
end_pc = 0x00007f040b6c1f39

$ disasm f3 0f 1e fa 90
Disassembling 0xf3 0x0f 0x1e 0xfa 0x90
llvm-mc:   0xf3 0x0f 0x1e 0xfa 0x90  endbr64 nop
capstone: 
bfd:       f3 0f 1e fa endbr64
DynamoRIO: f3 0f 1e fa nop %edx nop

(I should update my capstone copy -- I think they have support for this now)

These are not causing any fatal problem b/c they decode as nops to DR and it does not need to take any special action on them.