syscall gadgets missed for x86-64

[tezeb]

During recent Hitcon Quals, I've noticed that gadgets ending with syscall are not found. It does not work even with version 5.4 which is supposed to support it(#66).

$ ROPgadget --version
Version:        ROPgadget v5.4
Author:         Jonathan Salwan
Author page:    https://twitter.com/JonathanSalwan
Project page:   http://shell-storm.org/project/ROPgadget/
$ ROPgadget --binary vdso.dump | grep syscall
$ 

Running objump on the binary:

$ objdump -Mintel -D vdso.dump | grep -B 1 syscall
b5b:   b8 e4 00 00 00          mov    eax,0xe4
b60:   0f 05                   syscall 
--
edb:   4c 89 df                mov    rdi,r11
ede:   0f 05                   syscall 
$ 

It does not work regardless of Elf/raw mode.
Attached is vdso.so (base64 encoded):

vdso.b64.txt

[Benjamin Levy]

I think this is because syscall isn't considered a jump, so only syscall-ret gadgets are detected. This should be fixed because syscalls don't always return.