fix(multiboot): keep .mboot section even when not referenced

Otherwise, link-time garbage collection may throw away this section.

Author: mkroening

src/arch/x86_64/platform/multiboot/link.ld

@@ -7,7 +7,7 @@ SECTIONS
 {
   loader_start = phys;
   .mboot phys : AT(ADDR(.mboot)) {
-    *(.mboot)
+    KEEP(*(.mboot))
   }
   .text ALIGN(4096) : AT(ADDR(.text)) {
     *(.text)