[kernel] Rename kernel entrypoint

The kernel entrypoint being named _start conflicts with userspace
program entrypoints and makes debugging more difficult. Rename it to
_kernel_start.

src/arch/x86_64/kernel.ld

@@ -1,4 +1,4 @@
-ENTRY(_start)
+ENTRY(_kernel_start)
 SECTIONS
 {
 	. = 0xFFFF800000000000;