semihosting: Rename softmmu_FOO_user() -> uaccess_FOO_user()

Add a check in 'softmmu-uaccess.h' that the header is only
include in system emulation, and rename it as 'uaccess.h'.

Rename the API methods:

  - softmmu_[un]lock_user*() -> uaccess_[un]lock_user*()
  - softmmu_strlen_user() -> uaccess_strlen_user().

Update a pair of comments.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004090629.37473-9-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>