x86/elf: Use _BITUL() macro in UAPI headers

Replace BIT() in x86's UAPI header with _BITUL(). BIT() is not defined
in the UAPI headers and its usage may cause userspace build errors.

Fixes: 742c45c3

 ("x86/elf: Enumerate kernel FSGSBASE capability in AT_HWCAP2")
Signed-off-by: Joe Richey <joerichey@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210521085849.37676-2-joerichey94@gmail.com