On arm64 hosts, set host_cpu to 'arm64', not 'arm'.

The existing `substr()` was likely to make armv7 etc all map
to "arm". 64-bit arm hosts should get "arm64", to be consistent
with target_cpu.

At least on macOS, OperatingSystemArchitecture() returns "arm64",
not "aarch64".

Change-Id: I95b4d84040ad9984e303ced74a3a2685304bc78b
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/9261
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>

src/gn/args.cc

diff --git a/src/gn/args.cc b/src/gn/args.cc
index c40c7e1..4d5a720 100644
--- a/src/gn/args.cc
+++ b/src/gn/args.cc
@@ -342,10 +342,10 @@
     arch = kX86;
   else if (os_arch == "x86_64")
     arch = kX64;
+  else if (os_arch == "aarch64" || os_arch == "arm64")
+    arch = kArm64;
   else if (os_arch.substr(0, 3) == "arm")
     arch = kArm;
-  else if (os_arch == "aarch64")
-    arch = kArm64;
   else if (os_arch == "mips")
     arch = kMips;
   else if (os_arch == "mips64")