AArch64: Add support for memory protection keys

AArch64: Add support for memory protection keys

This patch adds support for memory protection keys on AArch64 systems with
enabled Stage 1 permission overlays feature introduced in Armv8.9 / 9.4
(FEAT_S1POE) [1].

 1. Internal functions "pkey_read" and "pkey_write" to access data
    associated with memory protection keys.
 2. Implementation of API functions "pkey_get" and "pkey_set" for
    the AArch64 target.
 3. AArch64-specific PKEY flags for READ and EXECUTE (see below).
 4. New target-specific test that checks behaviour of pkeys on
    AArch64 targets.
 5. This patch also extends existing generic test for pkeys.
 6. HWCAP constant for Permission Overlay Extension feature.

To support more accurate mapping of underlying permissions to the
PKEY flags, we introduce additional AArch64-specific flags. The full
list of flags is:

 - PKEY_UNRESTRICTED: 0x0 (for completeness)
 - PKEY_DISABLE_ACCESS: 0x1 (existing flag)
 - PKEY_DISABLE_WRITE: 0x2 (existing flag)
 - PKEY_DISABLE_EXECUTE: 0x4 (new flag, AArch64 specific)
 - PKEY_DISABLE_READ: 0x8 (new flag, AArch64 specific)

The problem here is that PKEY_DISABLE_ACCESS has unusual semantics as
it overlaps with existing PKEY_DISABLE_WRITE and new PKEY_DISABLE_READ.
For this reason mapping between permission bits RWX and "restrictions"
bits awxr (a for disable access, etc) becomes complicated:

 - PKEY_DISABLE_ACCESS disables both R and W
 - PKEY_DISABLE_{WRITE,READ} disables W and R respectively
 - PKEY_DISABLE_EXECUTE disables X

Combinations like the one below are accepted although they are redundant:

 - PKEY_DISABLE_ACCESS | PKEY_DISABLE_READ | PKEY_DISABLE_WRITE

Reverse mapping tries to retain backward compatibility and ORs
PKEY_DISABLE_ACCESS whenever both flags PKEY_DISABLE_READ and
PKEY_DISABLE_WRITE would be present.

This will break code that compares pkey_get output with == instead
of using bitwise operations. The latter is more correct since PKEY_*
constants are essentially bit flags.

It should be noted that PKEY_DISABLE_ACCESS does not prevent execution.

[1] https://developer.arm.com/documentation/ddi0487/ka/ section D8.4.1.4

Co-authored-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>