config.mak.uname: add support for clangarm64

CLANGARM64 is a relatively new MSYSTEM added by the MSYS2 team. In order
to have Git build correctly for this platform, let's add some
configuration for it to config.mak.uname.

Signed-off-by: Dennis Ameling <dennis@dennisameling.com>

config.mak.uname

@@ -697,6 +697,10 @@ 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