Fix x86 gadget discovery by decoupling alignment from step size

[iilegacyyii]

This patch fixes an issue where the --align option prevents discovery of valid x86/x64 gadgets such as pop rdi; ret.

ROPgadget currently uses the alignment value both as:

1. A constraint on the gadget start address
2. Step size when walking backward from a matched instruction

This coupling works for fixed-width ISAs (e.g. ARM Thumb, RISC-V compressed) but breaks on x86, where instruction lengths are variable and gadget starts often occur at ref - 1. With --align 2, these byte offsets are never examined, causing valid x86 gadgets to be missed.

Proposed fix:

1. Walks backward one byte at a time to inspect all possible gadget starts
2. Applies the alignment value only as a filter on the start address

This preserves the intended behavior for architectures where alignment is meaningful while restoring correct gadget enumeration on x86/x64.

Please note
I am currently unable to test this change on non-x86 architectures, so additional testing would be appreciated to ensure it does not introduce regressions on ARM, Thumb, or RISC-V. I can also provide a test ELF showcasing the issue if required.

[SweetVishnya]

@iilegacyyii, could you look into the failing tests, please?

[iilegacyyii]

Yep of course. Will take a look after work today.

[iilegacyyii]

@SweetVishnya hopefully fixed in 8bc16ae. Tested on my end and now the existing tests pass, and I am able to locate 2-byte gadgets with --align 2 set.

py ROPgadget.py --binary test-suite-binaries/elf-x64-bash-v4.1.5.1 --only "pop|ret" --align 2 --all | grep rdi

0x00000000004225ac : pop rdi ; ret
0x0000000000422868 : pop rdi ; ret
0x0000000000423438 : pop rdi ; ret
0x0000000000423d7a : pop rdi ; ret
0x0000000000425ed2 : pop rdi ; ret
0x000000000042ba42 : pop rdi ; ret
0x000000000042e4ca : pop rdi ; ret
0x000000000042e680 : pop rdi ; ret
0x000000000042e918 : pop rdi ; ret
0x00000000004331c4 : pop rdi ; ret
0x00000000004371b6 : pop rdi ; ret
0x0000000000438eac : pop rdi ; ret
0x00000000004390fa : pop rdi ; ret
0x00000000004391fa : pop rdi ; ret
0x0000000000439fa8 : pop rdi ; ret
0x000000000043c36a : pop rdi ; ret
0x000000000043c6e4 : pop rdi ; ret
0x000000000043ca2e : pop rdi ; ret
[...SNIPPED FOR BREVITY...]