arm: ensure the arch_name is always set for the build target

arm: ensure the arch_name is always set for the build target

This should never happen now if GCC is invoked by the driver, but in
the unusual case of calling cc1 (or its ilk) directly from the command
line the build target's arch_name string can remain NULL.  This can
complicate later processing meaning that we need to check for this
case explicitly in some circumstances.  Nothing should rely on this
behaviour, so it's simpler to always set the arch_name when
configuring the build target and be done with it.

gcc:

	* config/arm/arm.c (arm_configure_build_target): Ensure the target's
	arch_name is always set.