Move within_function to stabsread

Move within_function to stabsread

The global within_function is only used by a few symbol readers.  This
patch moves the global out of buildsym and into stabsread, which
seemed like a better fit.  It also arranges for the existing readers
to clear the global at the appropriate time.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* stabsread.h (within_function): Move from buildsym.h.
	* stabsread.c (start_stabs): Clear within_function.
	* coffread.c (coff_start_symtab): Clear within_function.
	* buildsym.h (within_function): Move to stabsread.h.
	* buildsym.c (prepare_for_building): Update.