[genmultilib] Update basic multilib configuration

The standard arm-eabi configuration comes with a basic set of multilibs that
are suitable mostly for simple testing of the compiler in various
configurations.  We try to keep the number of libraries build small so
that build times do not become too onerous.

Using the new auto-fp selection code we can now cover all supported
architectures except for those with single-precision only FP units with
just 4 multilibs.  This is done with the rewrite of t-arm-elf.  Now that we
canonicalize -mcpu into suitable -march definitions we don't need to match
CPU names to architectures any more; the driver will do this for us.

I also noticed whilst writing this patch that the existing MULTILIB_DEFAULTS
setting in the compiler was causing more problems than it was worth; and
furthermore was simply wrong if the compiler is ever configured with
--with-mode, --with-float or --with-endian.  The remaining options also
pertained to pre-eabi builds and aren't interesting today either.  It
seemed best to just delete the definition entirely.

	* config/arm/elf.h (MULTILIB_DEFAULTS): Delete.
	* config/arm/t-arm-elf: Rewritten.

From-SVN: r249294