Setting up all the infrastructure for BGEMM support in OpenBLAS, hopefully I found all the right places.

Derived mostly from the previous work done in https://github.com/OpenMathLib/OpenBLAS/pull/5287

Co-authored-by: Ye Tao <ye.tao@arm.com>

Force compilation with gfortran to use O0 on older Loongson hardware to avoid spurious test failures

There are two instances when building the tests where OpenBLAS fails to
build with OpenMP and clang due to library paths getting reset as flags
are set rather than appended. This seems to only affect certain
clang/libomp installations, but if it's already grabbing the correct
library paths we might as well use them.

Change-Id: Iae7800a32f5af3903c330882cdf6f292d885f266

This patch fixes compilation errors due to recent renaming from SH to SB
with BUILD_BFLOAT16.

This patch has Makefile changes to add test for shgemm which
compares sgemm and shgemm result.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>

When OpenBLAS is compiled with USE_OPENMP=1, it ignores OPENBLAS_NUM_THREADS and GOTO_NUM_THREADS flags.Therefore, you should use OMP_NUM_THREADS.

Without setting OMP_NUM_THREADS, a process will use maximal number of threads on a computing node. Thus, if there are 2 processes on the computing node, the thread will contend against other threads on CPU cores. As a result, the application will hang.