[whensun]

Description of the changes

This PR increases the ACE OpenSBI per-hart stack region from 8 KiB to 16 KiB to prevent stack exhaustion during security monitor initialization.

Previously, the default 8 KiB region reserved 4 KiB for OpenSBI scratch data, leaving only 4 KiB of usable stack space. During initialization with RISC-V Vector support, the observed stack usage reached 5,120 bytes, exceeding the available stack space and coinciding with intermittent security monitor startup panics.

This PR introduces:

• An ACE-specific ACE_OPENSBI_HART_STACK_SIZE constant set to 16 KiB.
• An increase in usable per-hart stack space from 4 KiB to 12 KiB.
• A platform-specific configuration without modifying the default OpenSBI stack size.

The change provides additional stack space for ACE security monitor initialization while keeping the modification limited to the ACE platform configuration.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Refactorization

How to test this PR?

1. Build ACE-RISCV with the updated OpenSBI hart stack configuration.
2. Verify that the per-hart stack region is configured to 16 KiB.
3. Run the security monitor with RISC-V Vector support enabled.
4. Verify that security monitor initialization completes without stack exhaustion or startup panics.

The stack exhaustion issue was observed during the development of Vector context-switch preservation in PR #145. The updated stack configuration was validated together with that implementation.

[wojciechozga]

Thank you for this contribution!