Try fs/gs_base registers before ptrace'ing

[lebr0nli]

fsbase() and gsbase() in pwndbg/regs.py will use ptrace by ourself to find segment base, but it doesn't work on remote debugging.

But GDB >= 8 can use fs_base and gs_base register to get the same result for x86-64 processes, and this feature works when remote debugging.

So by checking these two registers, even if we are using gdbserver to debug a x86-64 process, we can still use fsbase/gsbase without any problems if our GDB version >= 8

[codecov-commenter]

Codecov Report

Merging #1030 (82c95cd) into dev (e9b9ebe) will decrease coverage by 0.02%.
The diff coverage is 17.64%.

@@            Coverage Diff             @@
##              dev    #1030      +/-   ##
==========================================
- Coverage   49.88%   49.85%   -0.03%     
==========================================
  Files         157      157              
  Lines       18551    18564      +13     
  Branches     1498     1499       +1     
==========================================
+ Hits         9255     9256       +1     
- Misses       8998     9010      +12     
  Partials      298      298              

Impacted Files	Coverage Δ
pwndbg/commands/context.py	31.25% <0.00%> (-0.15%)	⬇️
pwndbg/regs.py	64.03% <0.00%> (-3.32%)	⬇️
pwndbg/color/disasm.py	80.39% <100.00%> (+0.39%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f2fcc95...82c95cd. Read the comment docs.

[disconnect3d]

@lebr0nli Thanks! Fyi I have refactored this a little bit in 7121936