Official support for aarch64 (#84)

* Fix hardcoded `armhf` in pmb/aportgen/binutils.py
* Generate aports: `binutils-aarch64`, `musl-aarch64`, `gcc-aarch64`
* Distccd: Remember the cross-compiler architecture (currently armhf
  or aarch64), that the current distccd is running as, and restart
  distccd with the correct architecture, in case a different arch
  is needed than what it is currently running as. (Depending on the
  cross-compiler arch, the PATH variable gets adjusted before
  starting distccd)
* Testcases: add aport generation for aarch64, add cross-compiling
  to aarch64
* pmb/parse/arch.py: Add aarch64 to the mapping

pmb/aportgen/binutils.py

@@ -39,7 +39,7 @@ def generate(args, pkgname):
 
     replace_functions = {
         "build": """
-            _target="$(arch_to_hostspec armhf)"
+            _target="$(arch_to_hostspec """ + arch + """)"
             cd "$builddir"
             "$builddir"/configure \\
                 --build="$CBUILD" \\