x86/vmlinux.lds.S: Take __START_KERNEL out conditional definition

If CONFIG_X86_32=y, the section start address is defined to be
"LOAD_OFFSET + LOAD_PHYSICAL_ADDR", which is the same as
__START_KERNEL_map.

Unify it with the 64-bit definition to simplify the code.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240313075839.8321-5-richard.weiyang@gmail.com