Support signed LDX loads in latest bpf_conformance

[yunwei37]

Summary

• add MEMSX signed LDX opcode definitions for ldxsb, ldxsh, and ldxsw
• lower signed LDX loads with sign extension in the LLVM JIT
• add unit coverage for signed byte/half-word/word stack loads
• update the latest bpf_conformance CI run to use --cpu_version v4 --include_groups callx and fail on regressions

Validation

• ./build/test/unit-test/llvm_jit_tests
• pinned spec suite: Passed 219 out of 219 tests.
• latest suite: Passed 313 out of 313 tests. with --cpu_version v4 --include_groups callx

Fixes #42
Fixes #43
Fixes #44

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.36%. Comparing base (bd82659) to head (16b171e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   75.17%   75.36%   +0.18%     
==========================================
  Files           5        5              
  Lines        1301     1311      +10     
  Branches      147      147              
==========================================
+ Hits          978      988      +10     
  Misses        323      323              

Flag	Coverage Δ
bpf_conformance	59.98% <30.76%> (-0.40%)	⬇️
spirv_opencl	39.25% <30.76%> (-0.23%)	⬇️
unittests	49.37% <100.00%> (+0.71%)	⬆️

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