patch 8.1.0422: cannot create map file with MinGW

Problem:    Cannot create map file with MinGW.
Solution:   Add support for $MAP. (Ken Takata, closes #3460)
diff --git a/src/Make_cyg_ming.mak b/src/Make_cyg_ming.mak
index ca0fa97..f381951 100644
--- a/src/Make_cyg_ming.mak
+++ b/src/Make_cyg_ming.mak
@@ -35,6 +35,9 @@
 # set to yes for a debug build
 DEBUG=no
 
+# set to yes to create a mapfile
+# MAP=yes
+
 # set to SIZE for size, SPEED for speed, MAXSPEED for maximum optimization
 OPTIMIZE=MAXSPEED
 
@@ -936,6 +939,10 @@
 LIB += -Wl,-Bstatic -lwinpthread -Wl,-Bdynamic
 endif
 
+ifeq (yes, $(MAP))
+LFLAGS += -Wl,-Map=$(TARGET).map
+endif
+
 all: $(TARGET) vimrun.exe xxd/xxd.exe install.exe uninstal.exe GvimExt/gvimext.dll
 
 vimrun.exe: vimrun.c