refactor(policy): centralize shared execution policy state

[prashymh]

Move shared execution-policy knobs out of scattered globals into a dedicated acs_execution_policy_t with explicit reset and accessor APIs. This keeps execution policy separate from run-request selection state, routes defaults and overrides through one place, and removes direct global and header coupling across VAL, PAL, baremetal, and UEFI paths.

• add acs_execution_policy.[ch] as the canonical shared policy object for print, PCIe, timeout, crypto, SLC, and EL1 trap settings
• replace direct policy g_* globals across VAL, PAL, test pool, baremetal, and UEFI code with policy fields or acs_policy_get_*() accessors
• update baremetal and UEFI entry paths to reset, populate, and consume execution policy explicitly
• split generic policy reset defaults from baremetal target overrides, and expose target defaults through acs_get_platform_execution_policy_defaults()
• update compile-time and EL3 override helpers to write execution policy separately from run-request filtering state
• remove duplicated policy declarations from PAL and UEFI headers and keep the shared policy API in one canonical header
• wire acs_execution_policy.c into the relevant VAL build files for baremetal and UEFI flows
• keep headers self-contained with shared fixed-width types and drop local scalar typedef wrappers that no longer add value
• retain documentation for sys_last_lvl_cache values and el1skiptrap_mask usage in the canonical policy definition

Change-Id: I1e320c46aaab9a6479b189d433a0a3d8f7a62258