Fix WSL alternate stack check (dotnet/coreclr#25972)

Fix WSL alternate stack check (dotnet/coreclr#25972)

* Fix WSL alternate stack check

On WSL, the alternate stack check in sigsegv_handler doesn't work. The
uc_stack members are always zero no matter whether the handler is
executed on an alternate or default stack. So the check to detect
whether we are running on an alternate stack or not is always returning
false on WSL and it prevents NULL reference exceptions from being
handled.
The fix is to introduce an env variable COMPlus_EnableAlternateStackCheck
that can be used to enable the alternate stack check. By default, the
sigsegv_handler is considered to always run on an alternate stack.

Commit migrated from https://github.com/dotnet/coreclr/commit/d7d8f91a8bad9effe9b5605edec6d26d540e0f85