cpu: softmax: enable relaxed acc_mode (fixes MFDNN-12822)

[dzarukin]

Fixes MFDNN-12822
Fixes #2237

Numbers are obtained not from the perf machine but the trend is visible:

perf,cpu,jit:avx10_1_512,,--mode=P --softmax --dir=FWD_I --sdt=bf16 --ddt=bf16 --stag=abcd --dtag=abcd --axis=3 --attr-acc-mode=relaxed 64x16x100x100,0,0.270752,0.192383,0,0.20202,0
perf,cpu,jit:avx10_1_512,,--mode=P --softmax --dir=FWD_I --sdt=bf16 --ddt=bf16 --stag=abcd --dtag=abcd --axis=3 64x16x100x100,0,0.208252,1.38135,0,1.44547,0

[dzarukin]

make test
disable benchdnn_all
enable benchdnn_softmax
enable benchdnn_nightly

[dzarukin]

Hi @ranukund, could you, please, review the change? Thanks.

[ranukund]

I've left a few comments. Please incorporate as you see fit. Thanks!