runtime: make systemstack tail call if already switched

Currently systemstack always calls its argument, even if we're already
on the system stack. Unfortunately, traceback with _TraceJump stops at
the first systemstack it sees, which often cuts off runtime stacks
early in profiles.

Fix this by performing a tail call if we're already on the system
stack. This eliminates it from the traceback entirely, so it won't
stop prematurely (or all get mushed into a single node in the profile
graph).

Change-Id: Ibc69e8765e899f8d3806078517b8c7314da196f4
Reviewed-on: https://go-review.googlesource.com/74050
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/runtime/export_test.go

@@ -395,3 +395,16 @@ func LockOSCounts() (external, internal uint32) {
 	}
 	return g.m.lockedExt, g.m.lockedInt
 }
+
+//go:noinline
+func TracebackSystemstack(stk []uintptr, i int) int {
+	if i == 0 {
+		pc, sp := getcallerpc(), getcallersp(unsafe.Pointer(&stk))
+		return gentraceback(pc, sp, 0, getg(), 0, &stk[0], len(stk), nil, nil, _TraceJumpStack)
+	}
+	n := 0
+	systemstack(func() {
+		n = TracebackSystemstack(stk, i-1)
+	})
+	return n
+}