[arm] Rework multilib support

[arm] Rework multilib support

It looks like the fuchsia port relied on inheriting the multilib rules from
the bare-metal port (the t-arm-elf makefile fragment), but that has now been
rewritten on the assuption that the base architecture is ARMv4t; fuchsia
has a base architecture of ARMv7-a.

To account for this, I've cloned the original t-arm-elf rules into a
new makefile fragment t-fuchsia and arranged for that to be used when
targetting this system.

	* config/arm/t-fuchsia: New file.
	* config.gcc (arm*-*-fuchsia*): Use it.

From-SVN: r249305