x86-64: align __PHYSICAL_START, remove __KERNEL_ALIGN

Handle the misconfiguration where CONFIG_PHYSICAL_START is
incompatible with CONFIG_PHYSICAL_ALIGN.  This is a configuration
error, but one which arises easily since Kconfig doesn't have the
smarts to express the true relationship between these two variables.
Hence, align __PHYSICAL_START the same way we align LOAD_PHYSICAL_ADDR
in <asm/boot.h>.

For non-relocatable kernels, this would cause the boot to fail.

[ Impact: fix boot failures for non-relocatable kernels ]

Reported-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>