Address Space Integrity: Add cred struct arch info

[paolonig]

From ww19 Safety Arch WG meeting it came out that we should also consider the settings of cred struct and its impact on the safety goal
ref: https://elixir.bootlin.com/linux/latest/source/include/linux/cred.h#L110

Impacted doc

wg-lfscs/address_space_integrity/address_space_integrity.md

Line 79 in 8d366de