patch 8.1.1119: no support for Windows on ARM64.

Problem:    No support for Windows on ARM64.
Solution:   Add ARM64 support (Leendert van Doorn)
diff --git a/src/GvimExt/Makefile b/src/GvimExt/Makefile
index 27ff953..a865989 100644
--- a/src/GvimExt/Makefile
+++ b/src/GvimExt/Makefile
@@ -25,6 +25,8 @@
 !  ifdef PLATFORM
 !   if ("$(PLATFORM)" == "x64") || ("$(PLATFORM)" == "X64")
 CPU = AMD64
+!   elseif ("$(PLATFORM)" == "arm64") || ("$(PLATFORM)" == "ARM64")
+CPU = ARM64
 !   elseif ("$(PLATFORM)" != "x86") && ("$(PLATFORM)" != "X86")
 !    error *** ERROR Unknown target platform "$(PLATFORM)". Make aborted.
 !   endif
@@ -56,11 +58,17 @@
 SUBSYSTEM = $(SUBSYSTEM),$(SUBSYSTEM_VER)
 !endif
 
+!if "$(CPU)" == "ARM64"
+OFFSET = 0x11C000000
+!else
+OFFSET = 0x1C000000
+!endif
+
 all: gvimext.dll
 
 gvimext.dll:    gvimext.obj	\
 		gvimext.res
-	$(link) $(lflags) -dll -def:gvimext.def -base:0x1C000000 -out:$*.dll $** $(olelibsdll) shell32.lib comctl32.lib -subsystem:$(SUBSYSTEM)
+	$(link) $(lflags) -dll -def:gvimext.def -base:$(OFFSET) -out:$*.dll $** $(olelibsdll) shell32.lib comctl32.lib -subsystem:$(SUBSYSTEM)
 	if exist $*.dll.manifest mt -nologo -manifest $*.dll.manifest -outputresource:$*.dll;2
 
 gvimext.obj: gvimext.h