Detect MIPS64 hosts as "mips" architecture

OCTEON II boards like the Edge Router Pro are actually MIPS64
processors.  So, platform.machine() reports them as "mips64".

BUG=

Review-Url: https://codereview.chromium.org/2061893002
Cr-Original-Commit-Position: refs/heads/master@{#399900}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a321aeb2364edb926de15e1486d1f98f84d96489

detect_host_arch.py

@@ -21,6 +21,8 @@ def HostArch():
     host_arch = 'x64'
   elif host_arch.startswith('arm'):
     host_arch = 'arm'
+  elif host_arch.startswith('mips'):
+    host_arch = 'mips'
 
   # platform.machine is based on running kernel. It's possible to use 64-bit
   # kernel with 32-bit userland, e.g. to give linker slightly more memory.