* make building the bfloat16 BLAS functions conditional on BUILD_HALF

* pass the BUILD_HALF option to gensymbol

* Pass BUILD_HALF as a compiler define for dynamic_arch builds

This patch adds support for bfloat16 data type matrix multiplication kernel.
For architectures that don't support bfloat16, it is defined as unsigned short
(2 bytes).  Default unroll sizes can be changed as per architecture as done for
SGEMM and for now 8 and 4 are used for M and N.  Size of ncopy/tcopy can be
changed as per architecture requirement and for now, size 2 is used.

Added shgemm in kernel/power/KERNEL.POWER9 and tested in powerpc64le and
powerpc64.  For reference, added a small test compare_sgemm_shgemm.c to compare
sgemm and shgemm output.

This patch does not cover OpenBLAS test, benchmark and lapack tests for shgemm.
Complex type implementation can be discussed and added once this is approved.

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

a fortran compiler, please try make ONLY_CBLAS=1.

This mode only compiler CBLAS without BLAS fortran interface and LAPACK.

For example, make USE_NETLIB_GEMV=1