Cygwin: Remove nostdlib Makefile variable

It's used in one place, and it's value is unconditional.

winsup/Makefile.common

@@ -30,8 +30,6 @@ top_builddir:=$(call justdir,${target_builddir})
 cygwin_build:=${target_builddir}/winsup/cygwin
 newlib_build:=${target_builddir}/newlib
 
-nostdlib:=-nostdlib
-
 VPATH:=${srcdir}
 
 .SUFFIXES:

winsup/cygwin/Makefile.in

@@ -679,7 +679,7 @@ $(LDSCRIPT): $(LDSCRIPT).in
 $(TEST_DLL_NAME): $(LDSCRIPT) dllfixdbg $(DLL_OFILES) $(LIBSERVER) $(LIBC) $(LIBM) $(API_VER) Makefile $(VERSION_OFILES)
 	$(CXX) $(CXXFLAGS) \
 	-mno-use-libstdc-wrappers \
-	-Wl,--gc-sections $(nostdlib) -Wl,-T$(firstword $^) -static \
+	-Wl,--gc-sections -nostdlib -Wl,-T$(firstword $^) -static \
 	-Wl,--heap=0 -Wl,--out-implib,cygdll.a -shared -o $@ \
 	-e $(DLL_ENTRY) $(DEF_FILE) $(DLL_OFILES) $(VERSION_OFILES) \
 	$(MALLOC_OBJ) $(LIBSERVER) $(LIBM) $(LIBC) \