riscv: add defconfigs

Follow the lead of the other arches and just set the TARGET variable.
Otherwise you get an error when running defconfig with ARCH=riscv{32,64}

Signed-off-by: Charles Mirabile <cmirabil@redhat.com>

extra/Configs/defconfigs/riscv32/defconfig

@@ -0,0 +1 @@
+TARGET_riscv32=y

extra/Configs/defconfigs/riscv64/defconfig

@@ -0,0 +1 @@
+TARGET_riscv64=y