[Local GC] Add GetThread to the GC/EE interface

[swgillespie]

This fixes a linker error when building the GC as a shared library.

[jkotas]

It it pretty unfortunate how many GCToEEInterface indirect calls the mode flips compile into now:

Consider the following fragment - it will compile into 3 indirect calls with local GC. Instead, it can be just a single call (both GetThread and IsPreemptiveGCDisabled check can be done as part EnablePreemptiveGC):

current_thread = GCToEEInterface::GetThread(); 
cooperative_mode = gc_heap::enable_preemptive (current_thread);

I think we should make IsPreemptiveGCDisabled, EnablePreemptiveGC, DisablePreemptiveGC, GetAllocContext, CatchAtSafePoint to always operate on current thread, and get rid of the thread argument on them. They are always used on the current thread and it will be cheaper to fetch the current thread internally in these methods.

Otherwise looks good.

[swgillespie]

@jkotas Thanks! Created issue https://github.com/dotnet/coreclr/issues/12043 to track that for a follow-up PR.

[swgillespie]

Rebased against master to fix a merge conflict.

[swgillespie]

@dotnet-bot test Tizen armel Cross Release Build
@dotnet-bot test Windows_NT x64 Debug Build and Test

[swgillespie]

thanks!