[arm] Rewrite t-aprofile using new selector methodology

Now that the default FPU is 'auto' we can finally rewrite (and
simplify) the rules for mapping compiler options to multilibs.  We
no-longer need to know the specific CPU, since the driver will
construct a suitable -march flag for us; this greatly simplifies the
overall logic.  This patch rewrites the library list for A-profile
cores.  We use various Make extention rules to simplify the logic even
further.

A couple of minor tweaks to the configure script and to the main
driver ensures that we always know the setting of -mfloat-abi and
-marm/-mthumb.  Again, this helps simplify the logic further.  The
change to arm_target_thumb_only relies on the fact that this routine
is only called if neither -marm nor -mthumb has been previously
selected or specified by the user.

A new testsuite module is added to check the libraries generated.  The
new tests are only run if the compiler is configured with the relevant
multilibs enabled.

gcc:
	* config.gcc: (arm*-*-*): When building a-profile libraries, force
	the driver to pass through the default setting of -mfloat-abi.
	* common/config/arm/arm-common.c (arm_target_thumb_only): Return -marm
	rather than NULL.
	* config/arm/t-multilib (MULTILIB_REUSE): Initialize to empty.
	(all_feat_combs): New rule.
	(MULTILIB_OPTIONS): Use explicit ARM and Thumb directories.  Rework
	default libraries.
	* config/arm/t-aprofile: Rewrite.

gcc/testsuite:
	* gcc.target/arm/multilibs.exp: New file.

From-SVN: r249296