Add x86_64 kernel bring-up with build-selected platform adapters

toolchain/x86_64/linker.script

@@ -0,0 +1,31 @@
+/* Copyright 2026 Aaron Hance
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+OUTPUT_FORMAT(elf64-x86-64)
+ENTRY(loader)
+SECTIONS
+{
+    /* Bootstrap-only low mapping. Final kernel address-space policy is a
+       separate milestone; do not inherit the i386 0xC0000000 offset. */
+    . = 0x100000;
+    kernel_start = .;
+    .multiboot : { KEEP(*(.multiboot)) }
+    .text ALIGN(4096) : { *(.text .text.*) }
+    .rodata ALIGN(4096) : { *(.rodata .rodata.*) }
+    .data ALIGN(4096) : { *(.data .data.*) }
+    load_end = .;
+    .bss ALIGN(4096) (NOLOAD) : { *(COMMON) *(.bss .bss.*) }
+    . = ALIGN(4096);
+    kernel_end = .;
+    ASSERT(kernel_end < 0x100000000, "Bootstrap must fit below 4 GiB")
+    /DISCARD/ : { *(.comment) *(.note*) *(.eh_frame*) }
+}