Fix regression SAXPY when compiler with OpenXL compiler.

SAXPY built with OpenXL regresses when compared to SAXPY
built with gcc. OpenXL compiler doesn't know that the
SAXPY inner kernel assembly is a 64 element loop and
to it the remainder loop is the main loop. It vectorizes
and interleaves the remainder to be a 48 elements per
iteration loop. With a max of 63 iterations, a 48 element
loop is mostly not going to get executed, so the 1 element
scalar loop that is the remainder after that is probably
mostly what gets executed.

This can be fixed by adding a pragma, loop interleave_count(2)
which will result in 8 element loop.

Signed-off-by: Amrita H S <amritahs@linux.vnet.ibm.com>

Loongarch64: Improving the Performance and Stability of dgemm

Add forgotten conditional uses of PREFETCH

CI: Add clang test for loongarch64

This fixes a (cross-)compilation/linker error for PRESCOTT
on Yocto.

Signed-off-by: Zoltán Böszörményi <zoltan.boszormenyi@xenial.com>

* Exit early if INCX <= 0

loongarch: Optimizing the performance of the GEMM on servers

The CPUID(leaf:2 or leaf:0x80000006) is not supported on some confidential
VMs. As a result the get_l2_size() returns the default 512M which brings
performance issues.

Introduce the environment variable OPENBLAS_L2_SIZE provided by the user
to get the l2 cache size.

Suggested-by: "Keshavamurthy, Anil S" <anil.s.keshavamurthy@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>

Initialize the registers to prevent sporadic errors.