Cannot choose correct md5 command when attempting to build on iOS

[Torrekie]

Describe the bug
In Makefiles, it reads both macOS and iOS as "Darwin", but there's no "md5" command on iOS, so it will print error log like below:

/bin/sh: md5: command not found
warning : could not find HASH (md5), needed to differentiate builds using different flags

On iOS, the command should be "md5sum" instead of "md5".

To Reproduce
Steps to reproduce the behavior:

1. Compile it on a jailbroken iOS device following README.md

Desktop (please complete the following information):

• OS: [iOS]
• Version [14.2]
• Compiler [gcc]
• Build system [Makefile]