fix: replace hardcoded 0xC0000000 in slab.c with hal_mm_phys_to_virt()

fix: replace hardcoded 0xC0000000 in slab.c with hal_mm_phys_to_virt()

slab.c used 'base + 0xC0000000U' to convert physical to virtual
addresses, which is x86 higher-half specific. This breaks on ARM,
RISC-V, and MIPS where the kernel virtual base differs.

Add hal_mm_phys_to_virt(), hal_mm_virt_to_phys(), and
hal_mm_kernel_virt_base() to the HAL mm interface with proper
implementations for x86 (0xC0000000 offset) and identity-mapped
stubs for ARM, RISC-V, MIPS.

Passes: make, cppcheck, QEMU smoke test.