arm64: use fixmap for text patching

When kernel text is marked as read only, it cannot be modified directly.
Use a fixmap to modify the text instead in a similar manner to
x86 and arm.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>