BACKPORT: arm64: Factor out PAN enabling/disabling into separate uaccess_* macros

This patch moves the directly coded alternatives for turning PAN on/off
into separate uaccess_{enable,disable} macros or functions. The asm
macros take a few arguments which will be used in subsequent patches.

Note that any (unlikely) access that the compiler might generate between
uaccess_enable() and uaccess_disable(), other than those explicitly
specified by the user access code, will not be protected by PAN.

Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Bug: 31432001
Change-Id: I75a410139d0756edab3210ee091fa5d047a22e04
(cherry picked from commit bd38967d406fb4f9fca67d612db71b5d74cfb0f5)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>