Update Android NDK llvm toolchains directory name (#2048)

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 😄 .

syncthing/build-syncthing.py

@@ -9,7 +9,7 @@
 PLATFORM_DIRS = {
     'Windows': 'windows-x86_64',
     'Linux': 'linux-x86_64',
-    'Darwin': 'darwin-x86-64',
+    'Darwin': 'darwin-x86_64',
 }
 
 # The values here must correspond with those in ../docker/prebuild.sh