Don't reload proc macros on fork

[Berrysoft]

I come across this problem recently, and see the mailing list of cygwin: https://inbox.sourceware.org/cygwin/TYCPR01MB10926F334CDC9344A89527583F89CA@TYCPR01MB10926.jpnprd01.prod.outlook.com/

TL;DR: rustc fails to execute the linker when proc macro DLLs are too many.

As we don't need to reload these them when executing the linker, I think it's OK to set FORK_NO_RELOAD for the loaded proc macro DLLs. It's a little hacky though. dlfork is a cygwin extension without documents, and it cannot set whether to reload a specific DLL on fork. If available, I prefer such an option for dlopen.

Not sure what does the rustc maintainers think about this workaround. I'll also ask for advice from the cygwin maintainers.

[rustbot]

r? @oli-obk

rustbot has assigned @oli-obk.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer