Mono: increase default stack size on s390x/ppc64le; support DOTNET_Thread_DefaultStackSize for setting stack size.

[tmds]

This increases the default stack size on s390x and ppc64le. Due to missing implementation for tail call optimization, the vmr build exhaust the stack while building the fsharp repo. With an 8MB stack, the build can complete.

Additionally this adds support for a user to control the stack size using the DOTNET_Thread_DefaultStackSize envvar.

Fixes #124368.

@jkotas @akoeplinger @hamarb123 ptal.

[tmds]

Per my comments in #124368, I also want to take a closer look at the Alpine/Musl (under ENSURE_PRIMARY_STACK_SIZE) implementation and probably suggest a change. That will be for another day/PR.

My main interest is to have this change so that the F# compiler doesn't run out of stack space while compiling fsharp repo during the .NET build on ppc64le and s390x (dotnet/fsharp#19286).

[hamarb123]

Should we consider just using getrlimit on coreclr/naot also on all unix platforms? macOS defaults to 8MB for main thread by default (from that value), but then 512kB for secondary threads - we currently override manually to 1.5MB, but maybe it makes sense to just use logic this everywhere (that way it's user-controllable, consistent across all .NET on Unix, etc.)? Thoughts @tmds @jkotas?

[jkotas]

Should we consider just using getrlimit on coreclr/naot also on all unix platforms? macOS defaults to 8MB for main thread by default (from that value), but then 512kB for secondary threads - we currently override manually to 1.5MB, but maybe it makes sense to just use logic this everywhere

I am hesitant to bump the default stack sizes further. This was discussed before in #98007

[tmds]

@jkotas @akoeplinger I've made changed based on your feedback. Please take another look.

[akoeplinger]

LGTM with small comments. please fix the PR description/title

[tmds]

I've updated the title/description.

Thanks for reviewing and feedback @akoeplinger and @jkotas!