xtensa: clean up assembly arguments in uaccess macros

Numeric assembly arguments are hard to understand and assembly code that
uses them is hard to modify. Use named arguments in __check_align_*,
__get_user_asm and __put_user_asm. Modify macro parameter names so that
they don't affect argument names. Use '+' constraint for the [err]
argument instead of having it as both input and output.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>