iphonesim HXCPP_ARM64: distinct LIBEXTRA/objdir, arm64 dll linker (follow-up to 3668203c)

[AxGord]

Summary

Rescoped after 3668203 — the core arm64 iOS simulator support
(-arch arm64 + -mios-simulator-version-min) landed upstream in
3668203, which supersedes the compiler-flag part of this PR. Upstream
master has been merged in and this branch's equivalent -target triple
sections were dropped in favor of upstream's approach.

What remains (8 insertions, 5 deletions across 2 files):

• Distinct LIBEXTRA for arm64-sim (finish-setup.xml): with
  HXCPP_ARM64 set, simulator libs currently get the plain .iphonesim
  suffix — colliding with the i386 name. This PR names them
  .iphonesim-arm64 so arm64/x86_64/i386 simulator binaries can coexist
  (matters for multi-slice workflows and for Lime's NDLL naming).
• Per-arch objdir (iphonesim-toolchain.xml): all simulator archs
  currently share obj/iphonesim/; switching arch without a clean
  rebuild mixes object files. arm64-sim gets obj/iphonesim…-arm64/.
• dll linker arm64 (iphonesim-toolchain.xml): the dll linker
  still only emits -arch i386/-arch x86_64 — with HXCPP_ARM64 it
  produces an i386 dylib. Rarely hit (sim builds are static_link by
  default), but wrong; this adds the arm64 case with the same
  if/unless guards as the compiler.

With HXCPP_ARM64 unset, everything is byte-identical to master.

We've been carrying arm64-sim support in our soccertutor/hxcpp fork for
~1.5 years, used in production OpenFL+AIR apps that ship to iPad; built
and tested on Apple Silicon (M1 Pro) against Xcode 15/16 SDKs, iOS
simulator runtimes 16/17/18. xmllint --noout confirms both files are
well-formed.

Use

haxelib run lime build ios -simulator -Dhxcpp_arm64
# or direct hxcpp:
haxe -cp <path> -cpp out -DHXCPP_ARM64 -Diphonesim

Context for follow-up

This is the foundation for an Apple Silicon support PR queued for
openfl/lime (NDLL fallback chain, auto-OpenAL backend selection),
which keys off the arm64-sim LIBEXTRA naming.

Original PR description (pre-3668203c)

The original scope added full arm64 simulator support via
-target arm64-apple-ios${version}-simulator compiler flags, separate
LIBEXTRA/objdir, and the dll linker arch. The compiler-flag half was
implemented upstream independently in 3668203 using
-arch arm64 + -mios-simulator-version-min, which achieves the same
correct LC_BUILD_VERSION simulator platform marking.

[MRK2009]

Now it's much better, I think it would be cool if ARM64 support for iOS simulator going to be implemented in lime.

[MRK2009]

This commit causes the pr have merge conflicts
3668203

[AxGord]

Conflicts resolved — merged master in (fc373fe). Since 3668203 implements the arm64 simulator compiler flags upstream (thanks!), I dropped this branch's equivalent -target sections and rescoped the PR to the remaining gaps: distinct .iphonesim-arm64 LIBEXTRA, per-arch sim objdir, and the arm64 case in the dll linker (which otherwise still emits -arch i386 under HXCPP_ARM64). Diff vs master is now +8/−5 across the same 2 files; PR description updated accordingly.