increase max atomic size to 16 on 64-bit platforms

[vtjnash]

I expect this to do very poorly on CI, but the goal is to see how badly.

Closes #41280

[vtjnash]

So win32 & win64 are missing the libatomic functions for this, and on linux32, llvm generates a call to the wrong function. Elsewhere it worked, but we've got some bad text in one of the error messages:

Test Failed at /buildworker/worker/tester_linuxaarch64/build/share/julia/test/intrinsics.jl:195
  Expression: Core.Intrinsics.atomic_pointermodify(p, swap, S(1), :sequentially_consistent)
    Expected: ErrorException("atomic_pointermodify: invalid pointer for atomic operation")
      Thrown: TypeError(:atomic_pointermodifytyped_store, "", Int128, 0x00000001)

[vtjnash]

Pleasing to see here that most of the tests that fail are the ones that check specifically whether this is supported yet. The i686 error is still present though:

      From worker 13:	JIT session error: Symbols not found: [ __sync_val_compare_and_swap_16, __sync_lock_test_and_set_16 ]