Restore zero-initialization of variables in generic ztrsm_utcopy

* Add AppleClang crossbuild to MacOS/arm64 DYNAMIC_ARCH

* add casts to disambiguate svwhilelt for clang

Sapphire Rapids sbgemm fix

This patch removes the warning messages related to unused variables in
sbgemm_kernel_power10.c.

Signed-off-by: Manjul Mohan <manjul@linux.vnet.ibm.com>

[WIP] Disable gcc's tree-vectorizer for x86_64 CGEMV

Use SVE kernel for SGEMM/DGEMM on Arm(R) Neoverse(TM) V1

Propagate SWITCH_RATIO to DYNAMIC_ARCH builds

This re-spins #3869 with some additional copy unrolling which helps maintain SYRK performance.

After #3868, the SVE kernels represent a pretty good boost.

This re-uses ARMV8SVE as a base and I'm going to incrementally move everything to use ARMV8SVE in additional patches (as well as fix up anything that's not already in ARMV8SVE).

Previously dynamic builds were either using the default SWITCH_RATIO
or one from the higher level architecture; this patch ensures the
dynamic builds can use this parameter as well.

Improve the performance of sbgemm_tcopy on neoversen2

Remove prefetches from SVE kernels

This is a precursor to enabling the SVE kernels for Arm(R) Neoverse(TM)
V1 which has 256-bit SVE. Testing revealed that the SVE kernel was
actually worse in some cases than the existing kernel which seemed odd -
removing these prefetches the underlying architecture seems to do a better job
😸

Unlike [dcz]scal, sscal still used the original GotoBLAS SSE code from scal_sse.S.
This code follows dscal as closely as possible, except for the inc_x > 1 code
for which a plain C loop is used much like the one in cscal.c, instead of an
adaptation of the SSE2 asm code of dscal.c (I tried but the performance wasn't
better than the plain C loop).

dscal: use ymm registers in Haswell microkernel

Using 256-bit registers in dscal makes this microkernel consistent with
cscal and zscal, and generally doubles performance if the vector fits in
L1 cache.