Kernel: Introduce basic pre-kernel environment

This implements a simple bootloader that is capable of loading ELF64
kernel images. It does this by using QEMU/GRUB to load the kernel image
from disk and pass it to our bootloader as a Multiboot module.

The bootloader then parses the ELF image and sets it up appropriately.
The kernel's entry point is a C++ function with architecture-native
code.

Co-authored-by: Liav A <liavalb@gmail.com>

Kernel/Prekernel/CMakeLists.txt

@@ -0,0 +1,28 @@
+set(SOURCES
+    boot.S
+    multiboot.S
+    init.cpp
+    UBSanitizer.cpp
+    ../MiniStdLib.cpp
+)
+
+if ("${SERENITY_ARCH}" STREQUAL "i686")
+    set(PREKERNEL_TARGET Prekernel32)
+else()
+    set(PREKERNEL_TARGET Prekernel64)
+endif()
+
+add_executable(${PREKERNEL_TARGET} ${SOURCES})
+
+target_link_options(${PREKERNEL_TARGET} PRIVATE LINKER:-T ${CMAKE_CURRENT_SOURCE_DIR}/linker.ld -nostdlib)
+set_target_properties(${PREKERNEL_TARGET} PROPERTIES LINK_DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/linker.ld)
+
+target_link_libraries(${PREKERNEL_TARGET} gcc supc++)
+
+add_custom_command(
+    TARGET ${PREKERNEL_TARGET} POST_BUILD
+    COMMAND ${TOOLCHAIN_PREFIX}objcopy -O elf32-i386 ${CMAKE_CURRENT_BINARY_DIR}/${PREKERNEL_TARGET} ${CMAKE_CURRENT_BINARY_DIR}/Prekernel
+    BYPRODUCTS ${CMAKE_CURRENT_BINARY_DIR}/Prekernel
+)
+
+install(FILES "${CMAKE_CURRENT_BINARY_DIR}/Prekernel" DESTINATION boot)