ARM: uaccess: consolidate uaccess asm to asm/uaccess-asm.h

Consolidate the user access assembly code to asm/uaccess-asm.h.  This
moves the csdb, check_uaccess, uaccess_mask_range_ptr, uaccess_enable,
uaccess_disable, uaccess_save, uaccess_restore macros, and creates two
new ones for exception entry and exit - uaccess_entry and uaccess_exit.

This makes the uaccess_save and uaccess_restore macros private to
asm/uaccess-asm.h.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>