x86: unify and correct the GDT_ENTRY() macro

Impact: None (cleanup only)

Merge the GDT_ENTRY() macro between arch/x86/boot/pm.c and
arch/x86/kernel/acpi/sleep.c and put the new one in
<asm-x86/segment.h>.

While we're at it, correct the bitmasks for the limit and flags.  The
new version relies on using ULL constants in order to cause type
promotion rather than explicit casts; this avoids having to include
<linux/types.h> in <asm-x86/segments.h>.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>