purego: add support for s390x

[gdams]

What type of issue is this addressing?

feature

What this PR does | solves

Implements the necessary changes to support linux/s390x (with CGO_ENABLED=1).

For now we can't support without cgo until golang/go#77449 has been addressed although I have a CL to implement the required changes.

[Copilot]

Pull request overview

This PR adds initial Linux/s390x support to the purego syscall + callback machinery (with CGO-enabled execution in CI), extending the existing multi-arch SysV implementation with s390x-specific assembly, struct handling, and a new CI job to exercise it under QEMU.

Changes:

• Add s390x syscall/callback assembly implementations and struct (aggregate) argument/return support.
• Extend build tags / runtime arch handling to include s390x (callback trampoline sizing, register counts, float32 handling).
• Add a GitHub Actions job to cross-compile and run tests for linux/s390x under QEMU.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
zcallback_s390x.s	Adds generated callback trampoline table for s390x.
wincallback.go	Extends generator to emit the s390x trampoline table.
syscall_sysv_stackArgs.go	Enables stackArgs-based callback argument handling for s390x.
syscall_sysv.go	Enables SysV path for linux/s390x and defines callback entry sizing.
syscall_cgo_linux.go	Updates unsupported-arch guard list and panic message to include s390x.
sys_unix_s390x.s	Implements s390x callbackasm1 ABI bridge into Go.
sys_s390x.s	Implements s390x syscall15X ABI bridge for RegisterFunc calls.
struct_s390x.go	Adds s390x small-aggregate (<=16B) struct argument/return handling.
internal/fakecgo/asm_s390x.s	Adds fakecgo crosscall2 for s390x.
func_test.go	Updates arch gating/skips to include s390x.
func.go	Adds s390x register counts and float32 packing/return handling.
callback_test.go	Enables callback tests on s390x with one test skipped for arg-count limits.
README.md	Documents linux/s390x as a Tier 2 platform.
.github/workflows/test.yml	Adds a linux/s390x test job using cross-compiler + QEMU.

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[hajimehoshi]

LGTM

@TotallyGamerJet PTAL

[TotallyGamerJet]

LGTM