Enable MSVC support for imatmul kernels

This change adds MSVC support for the imatmul kernels. As a result, they
are also aligned, which results in some additional diffs.

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Anton Bondarenko <anton.bondarenko@arm.com>