NFC: some cleanup in gc.c

[d-netto]

• Remove duplicate declaration of jl_gc_wait_for_the_world.
• Update some comments to reflect that marking is now multithreaded.
• Move eytzinger tree functions to staticdata.c, since it's no longer used by the GC.

[d-netto]

Seems like CI is green for x86_64-apple-darwin, even though it segfaulted on one of the GC tests (https://buildkite.com/julialang/julia-master/builds/23380#0187d477-c58a-4a4c-a308-e52113c14074):

From worker 2:	 13.133515 seconds (198 allocations: 17.070 KiB)
--
  | From worker 2:	  9.442715 seconds (195 allocations: 16.883 KiB)
  | From worker 2:	  8.452907 seconds (195 allocations: 16.883 KiB)
  | From worker 2:	 31.060096 seconds (6.81 k allocations: 490.418 KiB, 0.10% compilation time)
  | From worker 2:	 16.570991 seconds (195 allocations: 16.883 KiB)
  | From worker 2:	 13.747034 seconds (195 allocations: 16.883 KiB)
  | From worker 2:	 17.792633 seconds (195 allocations: 16.883 KiB)
  | From worker 2:	 48.112748 seconds (3.98 k allocations: 288.633 KiB)
  | From worker 2:	  4.057283 seconds (195 allocations: 16.883 KiB)
  | From worker 2:	  3.349197 seconds (195 allocations: 16.883 KiB)
  | From worker 2:
  | From worker 2:	[37292] signal (11.1): Segmentation fault: 11
  | From worker 2:	in expression starting at none:0
  | From worker 2:	  2.414891 seconds (195 allocations: 16.883 KiB)
  | From worker 2:	  9.823090 seconds (3.98 k allocations: 288.703 KiB)
  | gc                                               (2) \|    89.15 \|   0.00 \|  0.0 \|       5.84 \|  1858.36

[d-netto]

Seems fine after #49579.