purego: properly handle 64-bit integers on 32-bit arm

[xakep666]

What issue is this addressing?

Closes #489

What type of issue is this addressing?

bug

What this PR does | solves

This PR adds handling of 64-bit arguments on 32-bit ARM cpus according to AAPCS requirements. Exact points:

C.3 If the argument requires double-word alignment (8-byte), the NCRN is rounded up to the next even register number.
C.7 If the argument required double-word alignment (8-byte), then the NSAA is rounded up to the next double-word address.

Also it fixes handling of 64-bit return values for calls and callbacks on 32-bit platforms. Necessary assembly code is already present on 386 and I've added missing parts on arm.

Verification:

• 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) within larger changes started from issue described in All function/callback calls triggers SIGILL on ARMv5 cpus without FPU #490.

[Copilot]

Pull request overview

This PR targets ARM 32-bit ABI (AAPCS) correctness in purego, specifically ensuring 64-bit integer arguments are aligned/placed correctly and 64-bit return values are reconstructed correctly for both Go→C calls and C→Go callbacks.

Changes:

• Add ARM32-specific padding/alignment handling for 64-bit integer arguments during marshaling and callback argument decoding.
• Fix 64-bit return handling for calls and callbacks on 32-bit platforms (including ARM assembly trampoline support).
• Add ABI regression tests (C helpers + Go test cases) for previously failing ARM32 int64 patterns.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
testdata/abitest/abi_test.c	Adds C helpers to exercise ARM32 int64 alignment cases.
syscall_unix.go	Updates callback argument decoding to account for ARM32 int64 alignment and writes 64-bit callback results via helpers.
syscall_stackargs_s390x.go	Adds callbackArgs helpers for 64-bit results (arch-specific callbackArgs layout).
syscall_stackargs_ppc64le.go	Adds callbackArgs helpers for 64-bit results (arch-specific callbackArgs layout).
syscall_notstackargs.go	Adds 64-bit result helpers that split into two uintptrs on 32-bit platforms.
sys_unix_arm.s	Ensures callback trampoline returns both low/high words for 64-bit results via R0/R1.
func.go	Implements ARM32 padding logic for 64-bit args/returns and updates argument counting/validation.
func_test.go	Adds Go-side ABI tests that call the new C helpers and validate results.

Suppressed comments (1)

func.go:187

• In the float case, usesSlots is also derived from ty.Size() and uses truncating division, which becomes 0 for float32 on 64-bit platforms and miscounts float64 on 32-bit ARM (where it consumes 2 slots). This can cause the argument-limit guard to be wrong. Compute usesSlots from arg.Size() with ceil-division and advance floats by usesSlots when the value stays in float registers.

			case reflect.Float32, reflect.Float64:
				usesSlots := int(ty.Size() / unsafe.Sizeof(uintptr(0)))

				if isARMPaddingNeeded(ty, floats+stack) {
					usesSlots++
				}
				if floats < floatArgRegs {
					floats++
				} else {
					stack += usesSlots
				}

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