Simplify Makefile generation for cygwin

The rule to generate cygwin.c with a symbolic link from cygwin/cygwin.c can
actually be written to the generated Makefile on all platforms, as it causes
no problems if not used. It only "triggers" when a cygwin.o is part of the
Makefile macro ARCHOBJS, at which point suffix rules permit make to infer
that cygwin.o can be generated from cygwin.c, and cygwin.c from
cygwin/cygwin.c

The rule to generate LIBPERL_NONSHR is not used. It seems that it was never
used. It was added in July 2008 as part of commit 5f9145a:
    Cygwin build harmonization, remove cygwin/Makefile.SHs

but the rule added to the top level Makefile.SH doesn't seem to correspond
to any rule in the removed file cygwin/Makefile.SHs

Makefile.SH

@@ -745,6 +745,9 @@ done
 
 $spitshell >>$Makefile <<'!NO!SUBS!'
 
+cygwin.c: cygwin/cygwin.c
+	$(LNS) cygwin/cygwin.c
+
 globals$(OBJ_EXT): $(generated_headers)
 
 uudmap.h mg_data.h: bitcount.h
@@ -856,19 +859,6 @@ MINIPERLEXP		= miniperl
 perl5.def: $(MINIPERLEXP) makedef.pl $(CONFIGPM) $(SYM) $(SYMH) miniperl.map
 	$(MINIPERL) makedef.pl PLATFORM=os2 -DPERL_DLL=$(PERL_DLL) CC_FLAGS="$(OPTIMIZE)" > perl5.def
 
-!NO!SUBS!
-	;;
-cygwin)
-	$spitshell >>$Makefile <<'!NO!SUBS!'
-cygwin.c: cygwin/cygwin.c
-	$(LNS) cygwin/cygwin.c
-
-LIBPERL_NONSHR		= libperl$(LIB_EXT)
-
-$(LIBPERL_NONSHR): $(perllib_objs)
-	$(RMS) $(LIBPERL_NONSHR)
-	$(AR) rc $(LIBPERL_NONSHR) $(perllib_objs)
-
 !NO!SUBS!
 	;;
 esac