Add FP16 IGEMM support

This change introduces three new kernels
* `kai_lhs_imatmul_pack_x16p2vlx2_x16p_sme`
* `kai_rhs_imatmul_pack_kxn_x16p2vlx2b_x16_x16_sme`
* `kai_imatmul_clamp_f16_f16p2vlx2_f16p2vlx2_2vlx2vl_sme2_mopa`

These are used for indirect matmul, for 16-bit floating point data.

This change also adds unit testing for the FP16 imatmul kernels. The
code is written in a type agnostic manner, as to easily allow testing
for other data types with very low effort. This required the addition of
non-templated `read`/`write` functions, as to allow runtime-generic
access.

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

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