Move syscall-args to gef-extras/scripts

[hugsy]

• Did you use the latest version of GEF from dev branch?
• Is your bug specific to GEF (not GDB)? - Try to reproduce it running gdb -nx
• Did you search through the documentation first?
• Did you check issues (including
  the closed ones) - and the PR?

Step 1: Describe your environment

• Operating System / Distribution:
• Architecture:
• GEF version (including the Python library version) run version in GEF.

Step 2: Describe your problem

Steps to reproduce

is-syscall and syscall-args should be moved to GEF-Extras since they rely on external dependencies.

They can be easily re-integrated to the context pane using register_external_context_pane

Minimalist test case

// compile with gcc -fPIE -pic -o my_issue.out my_issue.c
int main(){ return 0; }

Observed Results

• What happened? This could be a description, log output, etc.

Expected results

• What did you expect to happen?

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