depends: Mention aarch64 as common cross-compile target

Author: laanwj

depends/README.md

@@ -23,7 +23,8 @@ Common `host-platform-triplets` for cross compilation are:
 - `i686-w64-mingw32` for Win32
 - `x86_64-w64-mingw32` for Win64
 - `x86_64-apple-darwin11` for MacOSX
-- `arm-linux-gnueabihf` for Linux ARM
+- `arm-linux-gnueabihf` for Linux ARM 32 bit
+- `aarch64-linux-gnu` for Linux ARM 64 bit
 
 No other options are needed, the paths are automatically configured.