linux-user: Extract target errno to 'target_errno_defs.h'

linux-user: Extract target errno to 'target_errno_defs.h'

We want to access the target errno indepently of the rest of the
linux-user code. Move the header containing the generic errno
definitions ('errno_defs.h') to 'generic/target_errno_defs.h',
create a new 'target_errno_defs.h' in each target which itself
includes 'generic/target_errno_defs.h'.

Suggested-by: Richard Henderson <[email protected]>
Reviewed-by: Laurent Vivier <[email protected]>
Reviewed-by: Richard Henderson <[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Message-Id: <20210708170550.1846343[email protected]>
Signed-off-by: Laurent Vivier <[email protected]>