unix: take address in assembly for Darwin syscall wrappers

In Go 1.17 we will introduce a register-based ABI on some
platforms, as well as ABI wrappers to bridge the ABIs. For Darwin
syscall wrappers, it needs to be called directly, instead of
through wrappers. Currently, it is written as that the syscall
functions are defined in assembly and their addresses are taken
from Go using funcPC. In Go 1.17 this will result in the address
of the ABI wrapper, which is undesired.

In the syscall package in the standard library we changed to use
a compiler intrinsic internal/abi.FuncPCABI0 to take the address
of the syscall function. But that is not available to this repo
and not available in older versions of Go. Here we take a
different approach: taking the address directly from assembly.
This also ensures we get the address of the defined syscall
function, not the ABI wrapper.

Updates golang/go#45702.

Change-Id: Ia7480d0fb0ca4fb9bf2f36d2deb1e3e5e4eb8284
Reviewed-on: https://go-review.googlesource.com/c/sys/+/317894
Trust: Cherry Mui <cherryyz@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>

unix/syscall_darwin_libSystem.go

@@ -7,7 +7,7 @@
 
 package unix
 
-import "unsafe"
+import _ "unsafe"
 
 // Implemented in the runtime package (runtime/sys_darwin.go)
 func syscall_syscall(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
@@ -25,10 +25,3 @@ func syscall_syscallPtr(fn, a1, a2, a3 uintptr) (r1, r2 uintptr, err Errno)
 //go:linkname syscall_rawSyscall syscall.rawSyscall
 //go:linkname syscall_rawSyscall6 syscall.rawSyscall6
 //go:linkname syscall_syscallPtr syscall.syscallPtr
-
-// Find the entry point for f. See comments in runtime/proc.go for the
-// function of the same name.
-//go:nosplit
-func funcPC(f func()) uintptr {
-	return **(**uintptr)(unsafe.Pointer(&f))
-}