power/bgemm: add BFloat16-in, BFloat16-out GEMM kernel for POWER10

Add BGEMM (BF16 input → BF16 output) for POWER10 by reusing the
existing SBGEMM kernel infrastructure. A -DBGEMM compile flag switches
only the store path; the xvbf16ger2pp MMA instruction and BF16 packing
routines are shared with SBGEMM unchanged.

Changes
-------
kernel/power/KERNEL.POWER10
  - Register BGEMM kernel and copy-routine targets, reusing the
    sbgemm_ncopy/tcopy sources (packing layout is identical).

kernel/power/sbgemm_kernel_power10.c
  - Under BGEMM: force v4sf_t to float so accumulators stay in
    float32; add STORE4_BF16/STORE2_BF16 macros (read BF16 C, widen,
    apply alpha*acc, convert back via xvcvspbf16, store); add
    f32_to_bf16_scalar() for m&1/n&1 tails; add BGEMM variants of
    all SAVE_ACC macros covering the full m/n tile hierarchy.

param.h
  - Add BGEMM_DEFAULT_UNROLL_M=16, UNROLL_N=8, P/Q/R blocking
    parameters for POWER10, matching the 16x8 kernel tile.

Unit test (utest/test_extensions/test_bgemm.c)
  - Uses SBGEMM as a trusted reference. 21 test cases cover all four
    transpose combinations, all m/n remainder paths, odd-k, alpha=0,
    beta=0/1, and the m>=32 fast path. Tolerance 0.01 to account for
    the one extra BF16 rounding on the BGEMM store.

Performance (POWER10, single-threaded, Transa=N Transb=N)
----------------------------------------------------------
Step=1 (sizes 1–200):
  - Sizes 1–80:   baseline ~12–131 MFlops (scalar fallback);
                  patch 200–46,000 MFlops (up to ~350,000x faster).
  - Sizes 81–200: patch 1x–35x faster with 16x8 MMA tile engaged.

Step=8 (sizes 8–1024):
  - Baseline peaks at ~5,200 MFlops; patch sustains 40,000–760,000
    MFlops for sizes 384–960. Average speedup ~100x.

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

Author: Amrita H S

kernel/power/KERNEL.POWER10

@@ -3,6 +3,22 @@
 #CGEMM_BETA = ../generic/zgemm_beta.c
 #ZGEMM_BETA = ../generic/zgemm_beta.c
 
+# BGEMM (BFloat16-in, BFloat16-out) for POWER10 using plain MMA (xvbf16ger2pp).
+# The kernel source is shared with SBGEMM; -DBGEMM is injected by Makefile.L3
+# to switch the SAVE_ACC macros to convert float32 accumulators back to BF16.
+# UNROLL_M=16 (A-panel, packed by ncopy_16), UNROLL_N=8 (B-panel, packed by
+# ncopy_8) — matches the 16x8 primary tile in sbgemm_kernel_power10.c.
+BGEMM_BETA      = ../generic/gemm_beta.c
+BGEMMKERNEL     = sbgemm_kernel_power10.c
+BGEMMINCOPY     = sbgemm_ncopy_16_power10.c
+BGEMMITCOPY     = sbgemm_tcopy_16_power10.c
+BGEMMONCOPY     = sbgemm_ncopy_8_power10.c
+BGEMMOTCOPY     = sbgemm_tcopy_8_power10.c
+BGEMMINCOPYOBJ  = bgemm_incopy$(TSUFFIX).$(SUFFIX)
+BGEMMITCOPYOBJ  = bgemm_itcopy$(TSUFFIX).$(SUFFIX)
+BGEMMONCOPYOBJ  = bgemm_oncopy$(TSUFFIX).$(SUFFIX)
+BGEMMOTCOPYOBJ  = bgemm_otcopy$(TSUFFIX).$(SUFFIX)
+
 SBGEMM_BETA = ../generic/gemm_beta.c
 SBGEMMKERNEL    = sbgemm_kernel_power10.c
 SBGEMMINCOPY    = sbgemm_ncopy_16_power10.c