runtime: print a stack trace at "morestack on g0"

Error like "morestack on g0" is one of the errors that is very
hard to debug, because often it doesn't print a useful stack trace.
The runtime doesn't directly print a stack trace because it is
a bad stack state to call print. Sometimes the SIGABRT may trigger
a traceback, but sometimes not especially in a cgo binary. Even if
it triggers a traceback it often does not include the stack trace
of the bad stack.

This CL makes it explicitly print a stack trace and throw. The
idea is to have some space as an "emergency" crash stack. When the
stack is in a really bad state, we switch to the crash stack and
do a traceback.

Currently only implemented on AMD64 and ARM64.

TODO: also handle errors like "morestack on gsignal" and bad
systemstack. Also handle other architectures.

Change-Id: Ibfc397202f2bb0737c5cbe99f2763de83301c1c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/419435
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/export_test.go

@@ -694,7 +694,7 @@ func G0StackOverflow() {
 		// The stack bounds for g0 stack is not always precise.
 		// Use an artificially small stack, to trigger a stack overflow
 		// without actually run out of the system stack (which may seg fault).
-		g0.stack.lo = sp - 4096
+		g0.stack.lo = sp - 4096 - stackSystem
 		g0.stackguard0 = g0.stack.lo + stackGuard
 		g0.stackguard1 = g0.stackguard0