We currently require `vec` to be aligned according to its total storage
size.  For example, a `vec<uint32_t, 8>` must be aligned on a 32 byte
boundary (32 = sizeof(uint32_t) * 8).  This comes from OpenCL, which
has the same requirement.  The largest required alignment is for
`vec<double, 16>`, which is typically 128 bytes.

However, this is problematic in SYCL because we want to support a
variety of host compilers.  The MSVC compiler has a limit of 64 byte
alignment for function parameters.  Therefore, the SYCL alignment
requirements makes it impossible to pass a `vec<double, 16>` as a
function parameter when compiling with MSVC.  This is particularly
problematic because many of the SYCL "builtin" math function take
`vec` as a parameter type.

We therefore propose weakening the alignment requirement to make it
possible to support MSVC.  Of course, implementations are still allowed
to align `vec` more highly if this results in more efficient code.

We considered removing the special alignment requirements entirely for
`vec` and leaving this as an implementation detail.  However, existing
code may depend on this alignment, so we opted for the most
conservative change that still allows MSVC to work.  Therefore, we
propose capping the required alignment to 64 bytes.

Add a non-normative note explaining why the guaranteed alignment is
limited to 64 bytes.