Add ARM cross-compilation to GitHub Actions CI

[etr]

Summary

• Add ARM32 (ARMv7 hard-float) and ARM64 (AArch64) cross-compilation testing to GitHub Actions CI
• Validates that libhttpserver builds correctly for ARM targets
• Uses x86_64 runners with cross-compilation toolchains (build-only, no test execution)

Changes

• Added two new matrix entries for ARM cross-compilation builds
• Added ARM toolchain installation step (gcc-arm-linux-gnueabihf, gcc-aarch64-linux-gnu)
• Added cached cross-compiled libmicrohttpd builds for ARM targets
• Updated configure step to pass --host flag for cross-compilation
• Skipped test execution and cppcheck for cross-compilation (binaries can't run on x86_64)

Test plan

• Verify ARM32 job runs successfully and builds complete
• Verify ARM64 job runs successfully and builds complete
• Confirm tests are skipped (expected behavior for cross-compilation)
• Verify libmicrohttpd caching works correctly for ARM builds

Closes #131

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.54%. Comparing base (3be9e7c) to head (37d31d1).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #363   +/-   ##
=======================================
  Coverage   70.54%   70.54%           
=======================================
  Files          28       28           
  Lines        1436     1436           
  Branches      570      570           
=======================================
  Hits         1013     1013           
  Misses         35       35           
  Partials      388      388           

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