Reorganize gc.c to prepare for thread-safe variant, with thread context hack.

[ArchRobison]

This is a continuation of #10724. It's a new PR because Github does not allow reopening a pull request after a forced push, which I learned the hard way 😦

The goal of this PR is still the same: modify gc.c in ways to prepare for making it thread-safe later. All changes in this PR are intended to be cosmetic and not impact functionality or performance.

The improvement over #10724 is the use of macros FOR_CURRENT_HEAP and FOR_EACH_HEAP for establishing thread context. In the thread-safe version, these will look something like:

#define FOR_CURRENT_HEAP {jl_thread_heap_t* current_heap = jl_thread_heap;
#define END }
#define FOR_EACH_HEAP for( int t=jl_n_threads; --t>=0; )  { jl_thread_heap_t* current_heap = jl_all_heaps[t];
/*}*/
#define HEAP(x) (current_heap->x)

In the current PR, these macros devolve in a way that retains the current logic, albeit with a dummy variable current_heap that is used to check that the macros are being used where needed (albeit no checking for correct use.) Any modern compiler will optimize away the dummy variable.

The other change over #10724 is that I put the finalizer stuff back in its original position. Because one thread can set up a finalizer, but another thread can ask for it to be executed, correct implementation is going to require more than just per-thread lists, so that's a battle for another day.

[StefanKarpinski]

The tabs here may make the build fail.

[ArchRobison]

I was wondering what the extra green meant. I guessed tabs, but I couldn't find them in my working copy. I'll poke around to figure out how to eradicate them.

[StefanKarpinski]

This out to work:

perl -i -ple 's/\t/    /g'

[ArchRobison]

The problem is convincing Github that the tabs are not there. When I check out my branch, src/gc.c has no tabs.

[Keno]

Are those actually hard tabs? I believe github also displays it like this for soft tabs.

[ArchRobison]

What is a soft tab?

[Keno]

Spaces.

[ArchRobison]

What's puzzling me is why it's displaying those lines differently than many others that I also indented by 4 spaces.

[Keno]

GitHub can't always detect changes within a line. E.g. below where you added a static declaration, the same green box exists.

[ArchRobison]

Patch is 100% tab-free ingredients. @carnaval - okay to commit?

[carnaval]

LGTM. The macros don't even look that hacky to me, it's pretty much all we need inside gc as far as I can tell. This + a real implementation of the thread contexts + a barrier around the collection and we should have something somewhat working.