Kernel: Actually set physical base pointer correctly in prekernel stage

I did a mistake and set the kernel_physical_base value to be just on
the actual linked kernel ELF start offset, while this value should
represent together with KERNEL_MAPPING_BASE the actual higher-half load
address.

By changing this value, we resolve a bug in which disabling KASLR
doesn't work and will cause the prekernel to hang on this statement:
```c++
VERIFY(kernel_load_base >= kernel_mapping_base + 0x200000);
```

Kernel/Prekernel/init.cpp

@@ -116,7 +116,7 @@ extern "C" [[noreturn]] void init()
  halt();
  __builtin_memcpy(kernel_program_headers, kernel_image + kernel_elf_header.e_phoff, sizeof(Elf_Phdr) * kernel_elf_header.e_phnum);
 
- FlatPtr kernel_physical_base = (FlatPtr)kernel_image;
+ FlatPtr kernel_physical_base = (FlatPtr)0x200000;
  FlatPtr default_kernel_load_base = KERNEL_MAPPING_BASE + kernel_physical_base;
 
  FlatPtr kernel_load_base = default_kernel_load_base;
@@ -153,7 +153,7 @@ extern "C" [[noreturn]] void init()
  FlatPtr kernel_mapping_base = kernel_load_base & ~(FlatPtr)0x3fffffff;
 
  VERIFY(kernel_load_base % 0x1000 == 0);
- VERIFY(kernel_load_base >= kernel_mapping_base + 0x200000);
+ VERIFY(kernel_load_base >= kernel_mapping_base + kernel_physical_base);
 
  int pdpt_flags = 0x3;