- Fixes wrong return type for `is_close`
- Adds stricter compiler flags for test files so we don't see the above
issue again
- Re-uses test helper functions between compare_sgemm_sbgemm/bgemm.c

This also improves the testing and generic kernel by re-using the BF16
conversion functions.

Built on top of https://github.com/OpenMathLib/OpenBLAS/pull/5357 and derived from https://github.com/OpenMathLib/OpenBLAS/pull/5287

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

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>

CMake: Pass `OpenMP` compiler and linker flags through CMake targets

Without failures due to existing files

[POWER] Problem with multi-threaded SBGEMM

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.

This patch enables bfloat16 kernels by default for POWER processors.
Tested on Linux POWER8, POWER9, POWER10 and AIX POWER10 systems.

* limit flang optimizations to -O2

* Prevent addition of a BOM to test input (which would distort the names of output files)