Add support for CLANGARM64 target (#3916)

**This requires an ARM64-machine with Windows 11 installed (which
supports x64 emulation for MSYS2)**

### The main idea

- Use the main MSYS2/git-sdk-64 setup, which works on Windows 11 on ARM
thanks to x64-emulation
- Configure the official `clangarm64` MSYS2 repo
- Install `mingw-w64-clang-aarch64-toolchain` which contains the
ARM64-native Clang compiler

Author: dscho

config.mak.uname

@@ -726,6 +726,10 @@ ifeq ($(uname_S),MINGW)
 		prefix = /mingw64
 		HOST_CPU = x86_64
 		BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
+        else ifeq (CLANGARM64,$(MSYSTEM))
+		prefix = /clangarm64
+		HOST_CPU = aarch64
+		BASIC_LDFLAGS += -Wl,--pic-executable,-e,mainCRTStartup
         else
 		COMPAT_CFLAGS += -D_USE_32BIT_TIME_T
 		BASIC_LDFLAGS += -Wl,--large-address-aware