riscv: Fixup difference with defconfig

Let's follow the origin patch's spirit:

The only difference between rv32_defconfig and defconfig is that
rv32_defconfig has  CONFIG_ARCH_RV32I=y.

This is helpful to compare rv64-compat-rv32 v.s. rv32-linux.

Fixes: 1b937e8f ("RISC-V: Add separate defconfig for 32bit systems")
Signed-off-by: NGuo Ren <guoren@linux.alibaba.com>
Signed-off-by: NGuo Ren <guoren@kernel.org>
Reviewed-by: NArnd Bergmann <arnd@arndb.de>
Tested-by: NHeiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220405071314.3225832-9-guoren@kernel.orgSigned-off-by: NPalmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Makefile

@@ -154,3 +154,7 @@ PHONY += rv64_randconfig
 rv64_randconfig:
 	$(Q)$(MAKE) KCONFIG_ALLCONFIG=$(srctree)/arch/riscv/configs/64-bit.config \
 		-f $(srctree)/Makefile randconfig
+
+PHONY += rv32_defconfig
+rv32_defconfig:
+	$(Q)$(MAKE) -f $(srctree)/Makefile defconfig 32-bit.config