runtime: make convTstring write barrier unreachable from throw

CL 581215 changed 'throw' so that instead of print(s) it called
a more complicated function, printpanicval, that statically
appeared to have convTstring in its call graph, even though this
isn't dynamically reachable when called with a string argument.

However, this caused the link-time static callgraph test to point
out that throw (which is called in nowritebarrierrec contexts
such as markgc) reaches a write barrier.

The solution is to inline and specialize the printpanicval
function for strings; it reduces to printindented.

Thanks to mpratt for pointing out that the reachability
check is on the fully lowered code, and is thus sensitive
to optimizations such as inlining.
I added an explanatory comment on the line that generates
the error message to help future users confused as I was.

Fixes golang/go#67274

Change-Id: Ief110d554de365ce4c09509dceee000cbee30ad9
Reviewed-on: https://go-review.googlesource.com/c/go/+/584617
Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/cmd/compile/internal/ssagen/nowb.go

@@ -174,6 +174,14 @@ func (c *nowritebarrierrecChecker) check() {
 				fmt.Fprintf(&err, "\n\t%v: called by %v", base.FmtPos(call.lineno), call.target.Nname)
 				call = funcs[call.target]
 			}
+			// Seeing this error in a failed CI run? It indicates that
+			// a function in the runtime package marked nowritebarrierrec
+			// (the outermost stack element) was found, by a static
+			// reachability analysis over the fully lowered optimized code,
+			// to call a function (fn) that involves a write barrier.
+			//
+			// Even if the call path is infeasable,
+			// you will need to reorganize the code to avoid it.
 			base.ErrorfAt(fn.WBPos, 0, "write barrier prohibited by caller; %v%s", fn.Nname, err.String())
 			continue
 		}

src/runtime/panic.go

@@ -1014,13 +1014,12 @@ func sync_fatal(s string) {
 // issue #67274, so as to fix longtest builders.
 //
 //go:nosplit
-//go:noinline
 func throw(s string) {
 	// Everything throw does should be recursively nosplit so it
 	// can be called even when it's unsafe to grow the stack.
 	systemstack(func() {
 		print("fatal error: ")
-		printpanicval(s)
+		printindented(s) // logically printpanicval(s), but avoids convTstring write barrier
 		print("\n")
 	})
 
@@ -1041,7 +1040,7 @@ func fatal(s string) {
 	// can be called even when it's unsafe to grow the stack.
 	systemstack(func() {
 		print("fatal error: ")
-		printpanicval(s)
+		printindented(s) // logically printpanicval(s), but avoids convTstring write barrier
 		print("\n")
 	})