target/riscv: Replace HOST_BIG_ENDIAN #ifdef with if() check

Replace preprocessor-time #ifdef with a compile-time check
to ensure all code paths are built and tested. This reduces
build-time configuration complexity and simplifies code
maintainability.

No functional change intended.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20251010134226.72221-14-philmd@linaro.org>
2 files changed