Bug 1515528 - Prefer MSVC when building for aarch64 windows. r=froydnj

Depends on D15264

Differential Revision: https://phabricator.services.mozilla.com/D15265

--HG--
extra : moz-landing-system : lando

build/moz.configure/toolchain.configure

@@ -839,6 +839,10 @@ def default_c_compilers(host_or_target, other_c_compiler=None):
     def default_c_compilers(host_or_target, target, toolchain_prefix,
                             android_clang_compiler, *other_c_compiler):
         if host_or_target.kernel == 'WINNT':
+            # Prefer MSVC for aarch64 for now.
+            if host_or_target.cpu == 'aarch64':
+                supported = types = ('msvc', 'clang-cl', 'gcc', 'clang')
+            else:
                 supported = types = ('clang-cl', 'msvc', 'gcc', 'clang')
         elif host_or_target.kernel == 'Darwin':
             types = ('clang',)