Mark GC API functions as non-allocating

These might enable/disable garbage collection, but they don't themselves
trigger a safepoint or allocate objects (in most cases).

This does raise an interesting point: What should we do with
`GC.safepoint()` and `GC.gc()`? They are technically not allocating,
but many users may want to prohibit it anyway since they can trigger
a GC pause.

src/allocfunc.jl

@@ -19,6 +19,10 @@ const known_nonalloc_funcs = (
     "jl_pop_handler", "ijl_pop_handler",
     "jl_f_typeof", "ijl_f_typeof",
     "jl_clock_now", "ijl_clock_now",
+    "jl_gc_enable", "ijl_gc_enable",
+    "jl_gc_disable_finalizers_internal", "ijl_gc_disable_finalizers_internal",
+    "jl_gc_is_in_finalizer", "ijl_gc_is_in_finalizer",
+    "jl_enable_gc_logging", "ijl_enable_gc_logging",
 )
 
 const known_alloc_with_throw_funcs = (
@@ -27,6 +31,7 @@ const known_alloc_with_throw_funcs = (
     "jl_f_is", "ijl_f_is",
     "jl_f_throw", "ijl_f_throw",
     "jl_f__svec_ref", "ijl_f__svec_ref",
+    "jl_gc_enable_finalizers_internal", "ijl_gc_enable_finalizers_internal",
 )
 
 function is_alloc_function(name, ignore_throw)

test/runtests.jl

@@ -16,6 +16,11 @@ function same_ccall()
     a,b
 end
 
+function toggle_gc()
+    GC.enable(false)
+    GC.enable(true)
+end
+
 @testset "AllocCheck.jl" begin
     @test length(check_allocs(mod, (Float64,Float64))) == 0
     @test length(check_allocs(sin, (Float64,); ignore_throw=false)) > 0
@@ -31,4 +36,6 @@ end
     @test length(check_allocs(first, (Core.SimpleVector,); ignore_throw = false)) == 3
     @test length(check_allocs(first, (Core.SimpleVector,); ignore_throw = true)) == 0
     @test length(check_allocs(time, ())) == 0
+
+    @test length(check_allocs(toggle_gc, (); ignore_throw=false)) == 0
 end