purego: use sync.Pool in syscall_syscall15X (#328)

This works around the stack growth issue in 05d8da4 by not marking the function as nosplit.
This commit uses a pool to cache syscall15Args instances. Otherwise a new instance
was allocated on the heap for each purego call.

Author: oliverbestmann

syscall_sysv.go

@@ -14,14 +14,17 @@ import (
 
 var syscall15XABI0 uintptr
 
-//go:nosplit
 func syscall_syscall15X(fn, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15 uintptr) (r1, r2, err uintptr) {
-	args := syscall15Args{
+	args := thePool.Get().(*syscall15Args)
+	defer thePool.Put(args)
+
+	*args = syscall15Args{
 		fn, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15,
 		a1, a2, a3, a4, a5, a6, a7, a8,
 		0,
 	}
-	runtime_cgocall(syscall15XABI0, unsafe.Pointer(&args))
+
+	runtime_cgocall(syscall15XABI0, unsafe.Pointer(args))
 	return args.a1, args.a2, 0
 }