[proposal] Freeze __ARM_ARCH macro up until Armv8.x-A

[vhscampos]

LLVM and gcc have an older definition of this macro, thus they don't currently follow the ACLE.

Starting from Armv9-a, numerical comparisons break. For instance, 805 (Armv8-5.a) is greater than 9 (Armv9-a).

Action: freeze the macro definition up until Armv8.x-A. Thus it must not be used from Armv9-A onwards. The ACLE spec must be updated to inform so.

However, the ACLE specification must inform users not to use or rely on this macro to control architecture features usage. They must use specific feature macros instead.