[arm] reset all multilib variables

NB.  This configuration does not build in GCC-7 and doesn't build now either.

This patch resets a couple of multlib variables which previously were
not cleared.

It almost certainly needs further work to make it use the new option
framework correctly, but since the library configurations are already
clearly wrong, it's not clear what the changes need to be.  In
particular it tries to build a hard-float library for ARM7TDMI in both
ARM and thumb modes, but ARMv4t does not support any floating-point
instructions; furthermore, GCC has never supported a hard-float thumb1
library.

	* config/arm/t-phoenix (MULTILIB_REUSE): Clear variable.
	(MULTILIB_REQUIRED): Likewise.

From-SVN: r249303