using elfdump in check-shlibs

[Richard PALO <richard%netbsd.org@localhost>]

As part of the work relieving tools/tools.SunOS.mk of non native Gnu tools, 
I've been using lately elfdump in check-shlibs-elf.mk instead of solely greadelf.

http://www.netbsd.org/~richard/elfdump-in-check-shlibs.diff

I'm curious if there are platforms other than SunOS which may find this useful.

With this patch, if TOOLS_PLATFORM.elfdump is defined, the it is preferred,
else the previous defaults work as would be expected.

Also, greadelf is no longer added to USE_TOOLS, which seemed to be arbitrarily
ignored for various reasons.

With recent binutils, greadelf is particularly long-winded (which may be a bug),
so consequently this speeds things up a bit.

would be nice to get this in prior to the freeze.
-- 
Richard PALO