add short-circuit logic to elfread.c

add short-circuit logic to elfread.c

If minimal symbols have already been read into a per-BFD object, then
a symbol reader can skip re-reading them.  This changes the ELF reader
to do so.

We only skip the work if the file is ELF+DWARF.  If it has stabs or
mdebug sections, then I think extra information is computed during the
minsym creation pass; and so we must still repeat it.  Eventually even
this will go away, once all symbol types have switched to being
progspace-independent.  In the meantime this has no negative effect --
it is just a missing optimization for a small set of users.

This change also required a somewhat non-obvious change to the OBJSTAT
accounting code.  If a symbol reader skips re-reading minimal symbols,
then the corresponding OBJSTAT will not be updated.  This leads to a
test failure in gdb.base/maint.exp.

To fix this, I've moved the needed stat field out of objfile and into
the per-BFD object.

2014-02-26  Tom Tromey  <tromey@redhat.com>

	* elfread.c (elf_read_minimal_symbols): Return early if
	minimal symbols have already been read.  Add "ei" parameter.
	(elf_symfile_read): Call elf_read_minimal_symbols earlier.
	* minsyms.c (prim_record_minimal_symbol_full): Update.
	* objfiles.h (struct objstats) <n_minsyms>: Move...
	(struct objfile_per_bfd_storage) <n_minsyms>: ... here.
	* symmisc.c (print_objfile_statistics): Update.