configure: Fix cross-building for RISCV host (v5)

Update $linux_arch to keep using the shared linux-headers/asm-riscv/
include path.

Fixes: e3e477c ("configure: Fix cross-building for RISCV host")
Reported-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
[rth: Missed v5, so now applying the diff between v4 and v5.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

configure

@@ -1726,6 +1726,9 @@ if test "$linux" = "yes" ; then
   mips64)
     linux_arch=mips
     ;;
+  riscv32|riscv64)
+    linux_arch=riscv
+    ;;
   *)
     # For most CPUs the kernel architecture name and QEMU CPU name match.
     linux_arch="$cpu"