[kutil] Make enum bitfields usable in other scopes

Changing the SFINAE/enable_if strategy from a type to a constexpr
function means that it can be defined in other scopes than the functions
themselves, because of function overloading. This lets us put everything
into the kutil::bitfields namespace, and make bitfields out of enums in
other namespaces. Also took the chance to clean up the implementation a
bit.

src/kernel/acpi_tables.h

@@ -84,7 +84,7 @@ enum class acpi_fadt_flags : uint32_t
 	hw_reduced_acpi	= 0x00100000,
 	low_pwr_s0_idle	= 0x00200000
 };
-IS_BITFIELD(acpi_fadt_flags);
+is_bitfield(acpi_fadt_flags);
 
 struct acpi_fadt
 {