arm64: consolidate rox page protection logic

Consolidate the arm64 decision making for the page protections used
for executable pages, used by both the trampoline code and the kernel
text mapping code.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/E1q9T3v-00EDmW-BH@rmk-PC.armlinux.org.uk


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>