Copy gnulib obstack files

Copy gnulib obstack files

This copies obstack.[ch] from gnulib, and updates the docs.  The next
patch should be applied if someone repeats the import at a later date.

include/
	PR gdb/17133
	* obstack.h: Import current gnulib file.
libiberty/
	PR gdb/17133
	* obstack.c: Import current gnulib file.
	* obstacks.texi: Updated doc, from glibc's manual/memory.texi.