Update Android NDK llvm toolchains directory name

[adamszewe]

The current setup uses:

PLATFORM_DIRS = {
    'Windows': 'windows-x86_64',
    'Linux': 'linux-x86_64',
    'Darwin': 'darwin-x86-64',
}

I think the Darwin config has been configured like this by mistake.
Trying to build the app fails because nativeBuild task fail because it cannot locate the toolchain.
Maybe in the past darwin-x86-64 worked but I see the toolchains under x86_64 directory now.
And with x86_64 the build works of course 😄 .