kernel: Binary code patches on x86.

This will be used to support SMAP.

Sponsored-by: https://liberapay.com/korli

Cleaned up ELF64 address handling.

* platform_allocate_elf_region() is removed, it is implemented in platform-
independent code now (ELF*Class::AllocateRegion). For ELF64 it is now
assumed that 64-bit addresses are mapped in the loader's 32-bit address space
as (address - KERNEL_BASE_64BIT + KERNEL_BASE).
* mapped_delta field from preloaded_*_image removed, now handled compile-time
using the ELF*Class::Map method.
* Also link the kernel with -z max-page-size=0x1000, removes the need for
2MB alignment on the data segment (not going to map the kernel with large
pages for the time being).

Align the start of the kernel .data to a 2MB boundary, prevents ld from merging the text and data PHDRs.

Fixed incorrect link address in kernel linker script.

x86_64: added linker script for the kernel.

* Based on the x86 one, changes made to match the default GCC x86_64 linker
script and correct kernel load address added.
* Not tested yet as the kernel doesn't fully compile.

Cleaned up ELF64 address handling.

* platform_allocate_elf_region() is removed, it is implemented in platform-
independent code now (ELF*Class::AllocateRegion). For ELF64 it is now
assumed that 64-bit addresses are mapped in the loader's 32-bit address space
as (address - KERNEL_BASE_64BIT + KERNEL_BASE).
* mapped_delta field from preloaded_*_image removed, now handled compile-time
using the ELF*Class::Map method.
* Also link the kernel with -z max-page-size=0x1000, removes the need for
2MB alignment on the data segment (not going to map the kernel with large
pages for the time being).

Align the start of the kernel .data to a 2MB boundary, prevents ld from merging the text and data PHDRs.

Fixed incorrect link address in kernel linker script.

x86_64: added linker script for the kernel.

* Based on the x86 one, changes made to match the default GCC x86_64 linker
script and correct kernel load address added.
* Not tested yet as the kernel doesn't fully compile.