ldscripts/kernel: set load base of 0

* 0463b95 removed a messy workaround ensuring the load address of
  the kernel was 0 due to riscv64's PIC kernel design breaking rv64
  booting.
* This is the proper fix to set the load address of our PIC kernel
  as 0 (triggering a dynamic position)

Change-Id: I010d70bef90c84ca0243601b86d79fbb60c38728
Reviewed-on: https://review.haiku-os.org/c/haiku/+/9301
Reviewed-by: Alexander von Gluck <[email protected]>
Reviewed-by: waddlesplash <[email protected]>

src/system/ldscripts/riscv64/kernel.ld

@@ -9,7 +9,8 @@ OUTPUT_ARCH(riscv)
 ENTRY(_start)
 SECTIONS
 {
-  . = 0x80000000 + SIZEOF_HEADERS;
+  /* riscv64 kernel is PIC */
+  . = 0x0 + SIZEOF_HEADERS;
 
   .interp         : { *(.interp) }
   .note.gnu.build-id : { *(.note.gnu.build-id) }