"__rust_probestack" undefined in loadable modules

[adamrk]

If a module contains a function with a large stack (example) it will fail to build as a loadable module on x86_64 with the following error:

  MODPOST Module.symvers
ERROR: modpost: "__rust_probestack" [drivers/char/rust_example_3.ko] undefined!
make[1]: *** [scripts/Makefile.modpost:111: Module.symvers] Error 1
make[1]: *** Deleting file 'Module.symvers'
make: *** [Makefile:1455: modules] Error 2

(This is based on #52)

I think the problem is that the symbols from compiler_builtins aren't being exported.

[adamrk]

Looks like this is resolved by rust-lang/rust#77885 which switches to inlining __rust_probestack on LLVM 11+.