Preserve callee-saved registers in threadentry_trampoline

[qmuntal]

threadentry_trampoline is called from glibc (that is, C code) when creating a new thread. When the thread exists, threadentry_trampoline return back to glibc, which can perform some additional cleanups.

We currently aren't honoring the Linux C ABI. It specify some registers as calle-saved which are clobbered by the Go code. This can cause unexpected behaviors when returning to glibc when the thread exists, as the C compiler assumes some registers will have the same value as before the threadentry_trampoline function was called, but they don't.

This is a precondition to merge #344.
Also, might fix #304.

[Copilot]

Pull Request Overview

This PR fixes ABI compliance issues in threadentry_trampoline functions across multiple architectures (amd64, arm64, loong64). The trampoline functions are called from C code (glibc) when creating threads, and must preserve callee-saved registers according to the Linux C ABI to prevent unexpected behavior when returning to glibc after thread exit.

Key Changes:

• Implements proper preservation and restoration of callee-saved registers in threadentry_trampoline for all architectures
• Adjusts stack frame management to accommodate saved registers
• Ensures X15 register is zeroed on amd64 as required by Go's ABIInternal

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
internal/fakecgo/trampolines_amd64.s	Uses existing PUSH/POP macros to save/restore callee-saved registers; zeros X15 for ABIInternal compliance
internal/fakecgo/trampolines_arm64.s	Manually saves/restores callee-saved registers R19-R28, F8-F15, and frame pointer/link register with adjusted stack frame
internal/fakecgo/trampolines_loong64.s	Manually saves/restores callee-saved registers R22-R31, F24-F31, and return address with adjusted stack frame

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[hajimehoshi]

looks good to me in general

[Copilot]

Pull Request Overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[TotallyGamerJet]

Besides the open comment this LGTM

[hajimehoshi]

LGTM, thanks!

[hajimehoshi]

Note that this didn't fix the NetBSD segv issue yet unfortunately.