* new bsd.own.mk variable OBJECT_FMT with values "a.out" or "ELF".
* use OBJECT_FMT in Makefile tests that depend on toolchain.
* Add new bsd.own.mk variable NOSHLIB that, if set indicates a given
port doesnt yet do shared libraries. Set on powerpc.
* Document new feature-test variables in src/share/mk/bsd.README.
Tested on i386, mips, fakedup Alpha environment.