robustify vxworks glimits.h overriding

robustify vxworks glimits.h overriding

The glimits.h overriding used in gcc/config/t-vxworks was fragile: the
intermediate file would already be there in a rebuild, and so the
adjustments would not be made, so the generated limits.h would miss
them, causing limits-width-[12] tests to fail on that target.

While changing it, I also replaced the modern $(cmd) shell syntax with
the more portable `cmd` construct.

for  gcc/ChangeLog

	* Makefile.in (T_GLIMITS_H): New.
	(stmp-int-hdrs): Depend on it, use it.
	* config/t-vxworks (T_GLIMITS_H): Override it.
	(vxw-glimits.h): New.