Add low-precision POWER10 gemm kernels (#467)

Details:
- This commit adds a new BLIS sandbox that (1) provides implementations 
  based on low-precision gemm kernels, and (2) extends the BLIS typed 
  API for those new implementations. Currently, these new kernels can 
  only be used for the POWER10 microarchitecture; however, they may 
  provide a template for developing similar kernels for other 
  microarchitectures (even those beyond POWER), as changes would likely 
  be limited to select places in the microkernel and possibly the 
  packing routines. The new low-precision operations that are now 
  supported include: shgemm, sbgemm, i16gemm, i8gemm, i4gemm. For more 
  information, refer to the POWER10.md document that is included in 
  'sandbox/power10'.

frame/3/bli_l3_ukr_prot.h

@@ -36,16 +36,18 @@
 // Define template prototypes for level-3 micro-kernels.
 //
 
-#define GEMM_UKR_PROT( ctype, ch, opname ) \
+#define GEMM_UKR_PROT( ctype, ch, opname ) GEMM_UKR_PROT2(ctype, ctype, ch, opname)
+
+#define GEMM_UKR_PROT2( ctype_in, ctype_out, ch, opname ) \
 \
 void PASTEMAC(ch,opname) \
      ( \
        dim_t               k, \
-       ctype*     restrict alpha, \
-       ctype*     restrict a, \
-       ctype*     restrict b, \
-       ctype*     restrict beta, \
-       ctype*     restrict c, inc_t rs_c, inc_t cs_c, \
+       ctype_out* restrict alpha, \
+       ctype_in*  restrict a, \
+       ctype_in*  restrict b, \
+       ctype_out* restrict beta, \
+       ctype_out* restrict c, inc_t rs_c, inc_t cs_c, \
        auxinfo_t* restrict data, \
        cntx_t*    restrict cntx  \
      );