build: replace hosted target with static linux and add kernel target

Co-authored-by: aider (openrouter/moonshotai/kimi-k2.6) <aider@aider.chat>

kernel_entry.S

@@ -0,0 +1,98 @@
+# Multiboot2 header and x86_64 long-mode bootstrap
+
+.section .multiboot2, "a"
+.align 8
+multiboot_header:
+    .long 0xe85250d6
+    .long 0
+    .long multiboot_header_end - multiboot_header
+    .long -(0xe85250d6 + 0 + (multiboot_header_end - multiboot_header))
+
+    .align 8
+    .word 0
+    .word 0
+    .long 8
+multiboot_header_end:
+
+.section .bss, "aw", @nobits
+.align 4096
+pml4:
+    .space 4096
+pdpt:
+    .space 4096
+pd:
+    .space 4096
+
+.align 16
+stack_bottom:
+    .space 16384
+stack_top:
+
+.section .text
+.code32
+.global _start
+.type _start, @function
+_start:
+    movl $stack_top, %esp
+
+    movl $pdpt, %eax
+    orl $0x03, %eax
+    movl %eax, pml4
+
+    movl $pd, %eax
+    orl $0x03, %eax
+    movl %eax, pdpt
+
+    movl $0x00000083, %eax
+    movl $pd, %edi
+    movl $512, %ecx
+1:
+    movl %eax, (%edi)
+    addl $0x200000, %eax
+    addl $8, %edi
+    loop 1b
+
+    movl $pml4, %eax
+    movl %eax, %cr3
+
+    movl %cr4, %eax
+    orl $0x20, %eax
+    movl %eax, %cr4
+
+    movl $0xC0000080, %ecx
+    rdmsr
+    orl $0x100, %eax
+    wrmsr
+
+    movl %cr0, %eax
+    orl $0x80000000, %eax
+    movl %eax, %cr0
+
+    lgdt gdt64_pointer
+    ljmp $0x08, $long_mode_start
+
+.code64
+long_mode_start:
+    movabsq $stack_top, %rax
+    movq %rax, %rsp
+
+    movabsq $__bss_start, %rdi
+    movabsq $__bss_end, %rcx
+    subq %rdi, %rcx
+    xorq %rax, %rax
+    rep stosb
+
+    call kernel_main
+    cli
+1:
+    hlt
+    jmp 1b
+
+.section .rodata
+.align 8
+gdt64:
+    .quad 0x0000000000000000
+    .quad 0x00209A0000000000
+gdt64_pointer:
+    .word gdt64_pointer - gdt64 - 1
+    .quad gdt64