refactor: replace hardcoded 0xC0000000 in elf.c and uaccess.c with hal_mm_kernel_virt...

refactor: replace hardcoded 0xC0000000 in elf.c and uaccess.c with hal_mm_kernel_virt_base()

Both elf.c and uaccess.c had X86_KERNEL_VIRT_BASE defined as
0xC0000000U inside #if x86 blocks. While technically correct
(only compiled on x86), this is inconsistent with the rest of the
codebase which now uses hal_mm_kernel_virt_base() everywhere.

Replace with the HAL abstraction for consistency and to ensure
the kernel virtual base is defined in exactly one place per arch.

Passes: make, cppcheck, QEMU smoke test.