loongarch64: Add loongarch64 architecture detection support.

Bug: 1361860
Change-Id: I432208fd78fb99140a97b3979a257f4e2e513807
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3886536
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Brian Ryner <bryner@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

detect_host_arch.py

@@ -34,6 +34,8 @@ def HostArch():
     host_arch = 's390'
   elif host_arch.startswith('riscv'):
     host_arch = 'riscv64'
+  elif host_arch.startswith('loongarch64'):
+    host_arch = 'loong64'
 
   if host_arch == 'arm64':
     host_platform = platform.architecture()