purego: support arm soft-float abi

[xakep666]

What issue is this addressing?

Closes #490

What type of issue is this addressing?

feature

What this PR does | solves

This PR improves support of arm architecture for CPUs without FPU and for linux distros built with softfp abi.

Implementation details:

• Pull runtime variable 'goarmsoftfp' to detect if soft-float ABI is used.
• Assembly trampolines skips load/store to FP registers if soft-float is detected. This was implemented in exactly same way how it's done in Go runtime.
• Apply integer passing rules for float arguments/return values.
• Run tests for GOARM=5 (no FPU) in test workflow and add some arm-specific test cases.

Possible concerns: a way soft-float ABI usage is detected. I've decided to import a variable from 'runtime' package via //go:linkname in a specific way to ensure that &purego.runtime_goarmsoftfp == &runtime.goarmsoftfp. Last one has //go:linkname too because it's widely used across assembly routines.

Verification:

• Added test running for with soft-float ABI to test workflow and added specific test cases.
• Verified on real ARMv5TE CPU (Marvell Kirkwood, no FPU) by @Hirador (co-author of commit, pin-pointed some places to fix).

[Copilot]

Pull request overview

Adds ARM soft-float ABI support to purego so callbacks and registered functions work correctly on ARM CPUs/distros without an FPU (e.g., GOARM=5 / softfp), addressing SIGILL crashes caused by unconditional VFP register save/restore and incorrect float argument handling.

Changes:

• Detect ARM soft-float ABI via runtime.goarmsoftfp and adjust argument/return handling so float32/float64 follow integer passing rules under soft-float.
• Update ARM assembly trampolines to skip VFP register load/store when soft-float is active.
• Extend ABI tests and CI workflow to exercise GOARM=5 (soft-float) under QEMU, including new ARM-specific ABI test cases.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
testdata/abitest/abi_test.c	Adds ARM-focused float64 ABI test functions for register vs stack passing.
syscall_unix.go	Adjusts callback argument restoration for float types when ARM soft-float ABI is detected.
sys_unix_arm.s	Skips saving/restoring VFP regs in callback trampoline when runtime.goarmsoftfp != 0.
sys_arm.s	Skips loading/saving VFP argument/return registers in syscall trampoline when soft-float is active.
internal/fakecgo/trampolines_arm.s	Skips saving/restoring VFP callee-saved regs for thread-entry trampoline under soft-float.
internal/fakecgo/asm_arm.s	Skips saving/restoring VFP callee-saved regs in crosscall2 under soft-float.
go_runtime.go	Adds linkname reference to runtime.goarmsoftfp for soft-float detection in Go code.
func.go	Implements soft-float detection and applies integer ABI rules for float args/returns on ARM soft-float.
func_test.go	Adds ABI tests covering unaligned float64 passing/return (registers vs stack).
.github/workflows/test.yml	Expands Linux ARM CI to also build/test GOARM=5 soft-float binaries under QEMU.

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[xakep666]

Looks like android/amd64 test should be restarted. I see OOM in job logs.

[hajimehoshi]

It seems ok to depend on the runtime variable goarmsoftfp.

LGTM

@TotallyGamerJet PTAL

[TotallyGamerJet]

LGTM. I do have some concern that this adds a linkname to another stdlib symbol that currently isn't marked as sticking around but since arm is tier 2 I think that's okay.

https://github.com/golang/go/blob/e9a161bda9c56e2116714785f49a83adcb3f8fd0/src/runtime/runtime2.go#L1464