runtime: Panic if newstack at runtime.acquireLockRank [1.15 backport]

[gopherbot]

@ALTree requested issue #40843 to be considered for backport to the next 1.15 minor release.

@gopherbot, please backport to Go 1.15, as requested by prattmic.

[aclements]

Backport justification: This causes random runtime panics in programs compiled without optimizations (which mostly comes up when compiling for debugging; for example, Delve compiles targets without optimizations automatically). The fix is also extremely low-risk.

This doesn't need a 1.14 backport because it's part of lock rank checking, which was introduced in 1.15.

[cagedmantis]

Approved because it is a critical issue without a workaround.

[gopherbot]

Change https://golang.org/cl/252339 mentions this issue: [release-branch.go1.15] runtime: fix panic if newstack at runtime.acquireLockRank

[gopherbot]

Closed by merging a269e5f to release-branch.go1.15.