runtime: move alloc_noheap call (pure refactor)

See the next commit, where this makes more sense.

src/runtime/gc.go

@@ -0,0 +1,10 @@
+package runtime
+
+// Shared code for the various garbage collectors.
+
+import "unsafe"
+
+// Special alloc function that should never actually be called.
+// It is used instead of normal alloc in //go:noheap functions, and must either
+// be optimized away or throw a linker error.
+func alloc_noheap(size uintptr, layout unsafe.Pointer) unsafe.Pointer

src/runtime/runtime.go

@@ -66,11 +66,6 @@ func llvm_sponentry() unsafe.Pointer
 //export strlen
 func strlen(ptr unsafe.Pointer) uintptr
 
-// Special alloc function that should never actually be called.
-// It is used instead of normal alloc in //go:noheap functions, and must either
-// be optimized away or throw a linker error.
-func alloc_noheap(size uintptr, layout unsafe.Pointer) unsafe.Pointer
-
 //export malloc
 func malloc(size uintptr) unsafe.Pointer