gas: Check for overflow on return column in version 1 CIE DWARF

gas: Check for overflow on return column in version 1 CIE DWARF

In version 1 of DWARF CIE format, the return register column is just a
single byte.  For targets with large numbers of DWARF registers, any
use of a register with a high number for the return column
will (currently) silently overflow giving incorrect DWARF.

This commit adds an error when the overflow occurs.

gas/ChangeLog:

	* dw2gencfi.c (output_cie): Error on return column overflow.
	* testsuite/gas/riscv/cie-rtn-col-1.d: New file.
	* testsuite/gas/riscv/cie-rtn-col-3.d: New file.
	* testsuite/gas/riscv/cie-rtn-col.s: New file.

Change-Id: I1809f739ba7771737ec012807f0260e1a3ed5e64