[vxworks] avoid mangling __STDC_VERSION_LIMITS_H__

The mangling of the macro name that guards limits.h from reinclusion
was mangling a c23-required macro as well.  Make the edit pattern
stricter.


for  gcc/ChangeLog

	* config/t-vxworks (vxw-glimits.h): Don't mangle c23-required
	__STDC_VERSION_LIMITS_H__ define.

gcc/config/t-vxworks

@@ -57,7 +57,7 @@ T_GLIMITS_H = vxw-glimits.h
 
 vxw-glimits.h: $(srcdir)/glimits.h
 	ID=`echo $(BASEVER_c) | sed -e 's/\./_/g'` && \
-	sed -e "s/_LIMITS_H__/_LIMITS_H__$${ID}_/" < $< > $@T
+	sed -e "s/_LIMITS_H___/_LIMITS_H__$${ID}_/" < $< > $@T
 	mv $@T $@
 
 # Arrange to "provide" a tailored version of stdint-gcc.h