environment: Consider MSVC/x64 as native on arm64

Just like is already done for MSVS/x86.

We're going to assume Windows >= 11 in this case too, i.e. OS support
for x86 emulation.

mesonbuild/environment.py

@@ -300,6 +300,8 @@ def detect_windows_arch(compilers: CompilersDict) -> str:
     for compiler in compilers.values():
         if compiler.id == 'msvc' and (compiler.target in {'x86', '80x86'}):
             return 'x86'
+        if compiler.id == 'msvc' and os_arch == 'arm64' and compiler.target == 'x64':
+            return 'x86_64'
         if compiler.id == 'clang-cl' and compiler.target == 'x86':
             return 'x86'
         if compiler.id == 'gcc' and compiler.has_builtin_define('__i386__'):