double buffer for gemm

[futz12]

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.88%. Comparing base (5154f22) to head (242c790). :warning: Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6400      +/-   ##
==========================================
- Coverage   95.89%   95.88%   -0.02%     
==========================================
  Files         841      842       +1     
  Lines      266337   266551     +214     
==========================================
+ Hits       255409   255572     +163     
- Misses      10928    10979      +51     

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[github-actions[bot]]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	15234680	15212648	-22032 :kissing_heart:
armhf	6220404	6210752	-9652 :kissing_heart:
aarch64	9525552	9524560	-992 :kissing_heart:

[nihui]

there is one interesting flops test in https://github.com/Tencent/ncnn/pull/6284