arch: arm64: Increase stack sizes for userspace with FPU

Increase ARM64 stack sizes to accommodate deeper call stacks in
userspace and SMP configurations when FPU_SHARING is enabled:

- PRIVILEGED_STACK_SIZE: 1024 → 4096 bytes (with FPU_SHARING)
- TEST_EXTRA_STACK_SIZE: 2048 → 4096 bytes (with FPU_SHARING)

The default 1KB privileged stack is insufficient for ARM64 userspace
syscalls when FPU context switching is enabled.

Symptom: Userspace tests crash with Data Abort (EC 0x24) near stack
boundaries during syscalls, particularly on SMP configurations where
multiple threads exercise FPU lazy switching.

Fixes previously failing CI test on fvp_base_revc_2xaem SMP variants:
- kernel.threads.dynamic
- Multiple userspace tests with FPU_SHARING enabled

Signed-off-by: Nicolas Pitre <[email protected]>

Author: Nicolas Pitre

arch/arm64/core/Kconfig

@@ -121,10 +121,15 @@ config MAIN_STACK_SIZE
 config IDLE_STACK_SIZE
 	default 4096
 
+config PRIVILEGED_STACK_SIZE
+	default 4096 if FPU_SHARING
+	default 2048
+
 config ISR_STACK_SIZE
 	default 4096
 
 config TEST_EXTRA_STACK_SIZE
+	default 4096 if FPU_SHARING
 	default 2048
 
 config SYSTEM_WORKQUEUE_STACK_SIZE