runtime: mark all Go symbols called from assembly in other packages

This marks all Go symbols called from assembly in other packages with
"go:linkname" directives to ensure they get ABI wrappers.

Now that we have this go:linkname convention, this also removes the
abi0Syms definition in the runtime, which was used to give morestackc
an ABI0 wrapper. Instead, we now just mark morestackc with a
go:linkname directive.

This was tested with buildall.bash in the default configuration, with
-race, and with -gcflags=all=-d=ssa/intrinsics/off. Since I couldn't
test cgo on non-Linux configurations, I manually grepped for runtime
symbols in runtime/cgo.

Updates #31230.

Change-Id: I6c8aa56be2ca6802dfa2bf159e49c411b9071bf1
Reviewed-on: https://go-review.googlesource.com/c/go/+/179862
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/signal_unix.go

@@ -369,6 +369,9 @@ func sigtrampgo(sig uint32, info *siginfo, ctx unsafe.Pointer) {
 //
 // The signal handler must not inject a call to sigpanic if
 // getg().throwsplit, since sigpanic may need to grow the stack.
+//
+// This is exported via linkname to assembly in runtime/cgo.
+//go:linkname sigpanic
 func sigpanic() {
 	g := getg()
 	if !canpanic(g) {
@@ -843,7 +846,11 @@ func signalstack(s *stack) {
 }
 
 // setsigsegv is used on darwin/arm{,64} to fake a segmentation fault.
+//
+// This is exported via linkname to assembly in runtime/cgo.
+//
 //go:nosplit
+//go:linkname setsigsegv
 func setsigsegv(pc uintptr) {
 	g := getg()
 	g.sig = _SIGSEGV