Move some code from buildsym to stabsread

Move some code from buildsym to stabsread

A few things that currently reside in buildsym.c turn out to be
specific to the stabs reader.  This patch moves these from
buildsym.[ch] to stabsread.[ch].

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

	* stabsread.h (HASHSIZE, hashname, symnum, next_symbol_text)
	(next_symbol_text_func): Move from buildsym.h.
	* stabsread.c (hashname): Move from buildsym.c.
	* buildsym.h (HASHSIZE, symnum, next_symbol_text)
	(next_symbol_text_func, hashname): Move to stabsread.h.
	* buildsym.c: Don't include bcache.h
	(hashname): Move to stasbread.c.