crimson/os: define device_spec_t as packed

crimson/os: define device_spec_t as packed

per C++ standard 20, alignas cannot be applied to scoped enum. despite
that there is a resolution to address this, see
https://cplusplus.github.io/CWG/issues/2354.html, it's not included in
C++20. so we have to use a different way to address

> runtime error: reference binding to misaligned address 0x610000008395
> for type 'device_type_t', which requires 4 byte alignment

otherwise the code would fail to compile with Clang-15, like:

> <source>:3:13: error: 'alignas' attribute cannot be applied to an enumeration

Signed-off-by: Kefu Chai <tchaikov@gmail.com>