internal/syscall/unix: use internal/abi.FuncPC for syscall wrapper

Following CL 313230, this is for internal/syscall/unix package.

Updates #45702.

Change-Id: Ie6d8c1923dfeae56896212393c5c2a6e257648d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/316649


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>