purego: support struct arguments in callbacks

[hajimehoshi]

What issue is this addressing?

Updates #225

What type of issue is this addressing?

feature

What this PR does | solves

Add getCallbackStruct implementations for amd64 and arm64 to decode struct arguments from the callback frame. Update callbackWrap to call getCallbackStruct instead of treating all struct arguments as CDecl.

The struct argument tests are restructured to run each test case with both RegisterLibFunc (calling C) and GoCallbackFunc (Go callback via NewCallback), ensuring round-trip correctness through the callback path.

(This change is also mostly developed by Claude Code)

[Copilot]

Pull request overview

This PR extends purego’s callback (Go function exported as a C-callable pointer via NewCallback) support to correctly handle struct arguments on amd64 and arm64 Unix platforms, and expands the test suite to validate struct argument round-trips.

Changes:

• Add ABI-aware decoding of struct callback arguments on amd64 (SysV) and arm64 (AAPCS64), with stub implementations on unsupported platforms.
• Expand struct_test.go to run struct-argument tests through both RegisterLibFunc and NewCallback-based callbacks, and add identity/round-trip struct tests.
• Add new C identity functions in testdata/structtest/struct_test.c to support round-trip validation from Go tests.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
testdata/structtest/struct_test.c	Adds C identity functions used for struct argument round-trip tests.
syscall_windows.go	Adds stub callback frame helpers needed for cross-platform compilation.
syscall_unix.go	Enables struct callback args and routes struct inputs through getCallbackStruct.
struct_test.go	Refactors tests to run via both lib calls and NewCallback, adding identity tests.
struct_amd64.go	Implements amd64 SysV struct decoding from callback frames.
struct_arm64.go	Implements arm64 struct decoding (HFA/non-HFA, regs/stack) from callback frames.
struct_arm.go	Adds stub getCallbackStruct (panics) for unsupported arch.
struct_386.go	Adds stub getCallbackStruct (panics) for unsupported arch.
struct_loong64.go	Adds stub getCallbackStruct (panics) for unsupported arch.
struct_ppc64le.go	Adds stub getCallbackStruct (panics) for unsupported arch.
struct_riscv64.go	Adds stub getCallbackStruct (panics) for unsupported arch.
struct_s390x.go	Adds stub getCallbackStruct (panics) for unsupported arch.

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