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

Fixes: e3e477c3 ("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>