semihosting: Split common_semi_flen_buf per target

semihosting: Split common_semi_flen_buf per target

We already have some larger ifdef blocks for ARM and RISCV;
split out common_semi_stack_bottom per target.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>