Add ls-silent binary

This is a patched version of the previous ls binary, which just exits
when run:

```
╭─vagrant@testing ~/scratch/radare2-testbins/elf (ls-silent)
╰─➤ radiff2 ls ls-silent
0x00004070 41574156415541545589fd534889f348 => 48c7c03c00000048c7c7000000000f05 0x00004070
```
```
╭─vagrant@testing ~/scratch/radare2-testbins/elf (ls-silent)
╰─➤ rasm2 -d 48c7c03c00000048c7c7000000000f05
mov rax, 0x3c
mov rdi, 0
syscall
```

This is useful for creating radare2 test cases in debug mode where
there's something to analyze, but ls doesn't output anything breaking
the tests.