Move c6x REGNO_REG_CLASS out of line

Move c6x REGNO_REG_CLASS out of line

I have a series of patches that hides call_used_regs from target-
independent code, a knock-on effect of which is that (public) target
macros can't use call_used_regs either.  This patch fixes the only
case in which that was a problem.

2019-09-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/c6x/c6x-protos.h (c6x_set_return_address): Declare.
	* config/c6x/c6x.h (REGNO_REG_CLASS): Move implementation to
	* config/c6x/c6x.c (c6x_regno_reg_class): ...this new function.

From-SVN: r275597