MIPS: DEC: Fix an int-handler.S CPU_DADDI_WORKAROUNDS regression

Fix a commit 3021773c ("MIPS: DEC: Avoid la pseudo-instruction in
delay slots") regression and remove assembly errors:

arch/mips/dec/int-handler.S: Assembler messages:
arch/mips/dec/int-handler.S:162: Error: Macro used $at after ".set noat"
arch/mips/dec/int-handler.S:163: Error: Macro used $at after ".set noat"
arch/mips/dec/int-handler.S:229: Error: Macro used $at after ".set noat"
arch/mips/dec/int-handler.S:230: Error: Macro used $at after ".set noat"

triggering with with the CPU_DADDI_WORKAROUNDS option set and the DADDIU
instruction.  This is because with that option in place the instruction
becomes a macro, which expands to an LI/DADDU (or actually ADDIU/DADDU)
sequence that uses $at as a temporary register.

With CPU_DADDI_WORKAROUNDS we only support `-msym32' compilation though,
and this is already enforced in arch/mips/Makefile, so choose the 32-bit
expansion variant for the supported configurations and then replace the
64-bit variant with #error just in case.

Fixes: 3021773c

 ("MIPS: DEC: Avoid la pseudo-instruction in delay slots")
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: stable@vger.kernel.org # 4.8+
Patchwork: https://patchwork.linux-mips.org/patch/16893/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>