objc/NuOperators.m: some #if-conditional code needed to build on ARM64

[cho-m]

Seen while fixing Homebrew ARM64 build: Homebrew/homebrew-core#112070

Specifically, some code inside #if defined(__x86_64__) || TARGET_OS_IPHONE sections are needed to build a working mininush otherwise subsequent build steps output:

==> ./mininush tools/nuke
Nu uncaught exception: NuIvarAddedTooLate: explicit instance variables must be added when a class is created and before any method declarations
  from tools/nuke:137: in ivar
  from <TopLevel>:-1: in class
  from <TopLevel>:1: in progn

Not entirely sure what conditional is needed so opened issue rather than PR. In Homebrew, I just added defined(__arm64__) to compile the code.

---

e.g.

nu/objc/NuOperators.m

Lines 1798 to 1800 in 1b0cd7f

	#if defined(__x86_64__) || TARGET_OS_IPHONE
	Class newClass = nil;
	#endif

nu/objc/NuOperators.m

Line 1814 in 1b0cd7f

#if defined(__x86_64__) || TARGET_OS_IPHONE

nu/objc/NuOperators.m

Line 1856 in 1b0cd7f

#if defined(__x86_64__) || TARGET_OS_IPHONE