[release-branch.go1.15] runtime: fix panic if newstack at runtime.acquireLockRank

Process may crash becaues acquireLockRank and releaseLockRank may
be called in nosplit context. With optimizations and inlining
disabled, these functions won't get inlined or have their morestack
calls eliminated.
Nosplit is not strictly required for lockWithRank, unlockWithRank
and lockWithRankMayAcquire, just keep consistency with lockrank_on.go
here.

Updates #40843.
Fixes #40845.

Change-Id: I5824119f98a1da66d767cdb9a60dffe768f13c81
GitHub-Last-Rev: 38fd3ccf6e
GitHub-Pull-Request: golang/go#40844
Reviewed-on: https://go-review.googlesource.com/c/go/+/248878
Reviewed-by: Dan Scales <danscales@google.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
(cherry picked from commit b246c0e12f)
Reviewed-on: https://go-review.googlesource.com/c/go/+/252339
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>

src/runtime/lockrank_off.go

@@ -18,19 +18,29 @@ func getLockRank(l *mutex) lockRank {
 	return 0
 }
 
+// The following functions may be called in nosplit context.
+// Nosplit is not strictly required for lockWithRank, unlockWithRank
+// and lockWithRankMayAcquire, but these nosplit annotations must
+// be kept consistent with the equivalent functions in lockrank_on.go.
+
+//go:nosplit
 func lockWithRank(l *mutex, rank lockRank) {
 	lock2(l)
 }
 
+//go:nosplit
 func acquireLockRank(rank lockRank) {
 }
 
+//go:nosplit
 func unlockWithRank(l *mutex) {
 	unlock2(l)
 }
 
+//go:nosplit
 func releaseLockRank(rank lockRank) {
 }
 
+//go:nosplit
 func lockWithRankMayAcquire(l *mutex, rank lockRank) {
 }