semihosting: Move exec/softmmu-semi.h to semihosting/softmmu-uaccess.h

We have a subdirectory for semihosting; move this file out of exec.
Rename to emphasize the contents are a replacement for the functions
in linux-user/bsd-user uaccess.c.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>