runtime: remove unneeded noinline directives

Now that mid-stack inlining reports backtraces correctly, we no
longer need to protect against inlining in a few critical areas.

Update #19348
Update #28640
Update #34276

Change-Id: Ie68487e6482c3a9509ecf7ecbbd40fe43cee8381
Reviewed-on: https://go-review.googlesource.com/c/go/+/195818Reviewed-by: David Chase <drchase@google.com>

src/runtime/extern.go

@@ -200,7 +200,6 @@ func Caller(skip int) (pc uintptr, file string, line int, ok bool) {
 // directly is discouraged, as is using FuncForPC on any of the
 // returned PCs, since these cannot account for inlining or return
 // program counter adjustment.
-//go:noinline
 func Callers(skip int, pc []uintptr) int {
 	// runtime.callers uses pc.array==nil as a signal
 	// to print a stack trace. Pick off 0-length pc here

src/runtime/stack_test.go

@@ -599,9 +599,6 @@ func (s structWithMethod) callers() []uintptr {
 	return pc[:Callers(0, pc)]
 }
 
-// The noinline prevents this function from being inlined
-// into a wrapper. TODO: remove this when issue 28640 is fixed.
-//go:noinline
 func (s structWithMethod) stack() string {
 	buf := make([]byte, 4<<10)
 	return string(buf[:Stack(buf, false)])