feat: add native Android (Termux) support

[xprobo]

Summary

This PR adds seamless Android build and compilation support via Termux while preserving compatibility with existing macOS, Linux, and Windows environments.

Changes

Makefile

• Added architecture detection for aarch64 and armv7 to enable hardware cryptographic extensions where available.
• Added support for $PREFIX paths to improve dependency detection in Termux environments.

setup.sh

• Added environment detection for Termux.
• Uses pkg natively when running under Termux while retaining existing package manager behavior on other platforms.

random.hpp

• Added Android-specific handling using the __ANDROID__ macro.
• Bypasses the Bionic libc getrandom() wrapper by calling syscall(SYS_getrandom) directly when building on Android.
• Preserves the existing implementation and behavior for standard Linux distributions.

README.md

• Added installation and build instructions for Termux.
• Included Android-specific setup steps and required dependencies.

Compatibility

This change is fully backward compatible and does not modify the build process for existing macOS, Linux, or Windows users.