gdb: move some stabs functions to gdb/buildsym-legacy.h

The gdb/stabsread.h and .c files define 2 things that, while originally
intended only for stabs reading, actually end up being used for coff,
ecoff and maybe more debuginfo formats. That is the function "hashname",
and the macro HASHSIZE.  Both are used for small hashtables when reading
some symbols with incomplete information.

With the upcoming removal of stabs code, this code should be moved
somewhere, and the location that looked most reasonable was
gdb/buildsym-legacy.  No change in behavior is expected after this
commit.

Approved-By: Tom Tromey <tom@tromey.com>