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>

stubs/semihost.c

@@ -1,9 +1,9 @@
 /*
- * Semihosting Stubs for SoftMMU
+ * Semihosting Stubs for system emulation
  *
  * Copyright (c) 2019 Linaro Ltd
  *
- * Stubs for SoftMMU targets that don't actually do semihosting.
+ * Stubs for system targets that don't actually do semihosting.
  *
  * SPDX-License-Identifier: GPL-2.0-or-later
  */