Add iOS build support and improve macOS build configuration

[fosple]

This update introduces iOS build support and fixes broken macOS build process.

Key changes:

• iOS support:

  • Added build_ios.sh script to build for macOS, iOS device, and iOS simulator, producing an XCFramework.
  • Configured CMake to disable shared libraries and LTO for iOS builds (fixes incompatibility with XCFramework, disables bitcode)

• macOS build improvements:

  • Set default installation prefix to ~/Library/libg722 instead of /usr/local.
  • Disabled bitcode for macOS builds.
  • Updated Makefile to match new default install prefix logic.

• Documentation updates:

  • Expanded README.md with macOS and iOS build instructions.
  • Added environment variable instructions for macOS dynamic linking.

• Code adjustments for macOS:

  • Added endian conversion macros for Apple platforms in test.c.

• Miscellaneous:

  • Updated .gitignore to exclude build/* directories.

These changes make it easy to build and deploy libg722 for Apple platforms, including universal binaries and XCFramework packaging for iOS.