riscv: errata: andes: Makefile: Fix randconfig build issue

Compile the andes errata with cflags set to " -mcmodel=medany"
when CONFIG_RISCV_ALTERNATIVE_EARLY is enabled.
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202309111311.8tcq3KVc-lkp@intel.com/Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://lore.kernel.org/r/20230925153844.26820-1-prabhakar.mahadev-lad.rj@bp.renesas.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/errata/andes/Makefile

+ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
+CFLAGS_errata.o := -mcmodel=medany
+endif
+
 obj-y += errata.o